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Diffstat (limited to 'nptl/pthread_rwlock_common.c')
| -rw-r--r-- | nptl/pthread_rwlock_common.c | 924 |
1 files changed, 924 insertions, 0 deletions
diff --git a/nptl/pthread_rwlock_common.c b/nptl/pthread_rwlock_common.c new file mode 100644 index 0000000000..256508ca2a --- /dev/null +++ b/nptl/pthread_rwlock_common.c @@ -0,0 +1,924 @@ +/* POSIX reader--writer lock: core parts. + Copyright (C) 2016-2017 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + <http://www.gnu.org/licenses/>. */ + +#include <errno.h> +#include <sysdep.h> +#include <pthread.h> +#include <pthreadP.h> +#include <sys/time.h> +#include <stap-probe.h> +#include <atomic.h> +#include <futex-internal.h> + + +/* A reader--writer lock that fulfills the POSIX requirements (but operations + on this lock are not necessarily full barriers, as one may interpret the + POSIX requirement about "synchronizing memory"). All critical sections are + in a total order, writers synchronize with prior writers and readers, and + readers synchronize with prior writers. + + A thread is allowed to acquire a read lock recursively (i.e., have rdlock + critical sections that overlap in sequenced-before) unless the kind of the + rwlock is set to PTHREAD_RWLOCK_PREFER_WRITERS_NONRECURSIVE_NP. + + This lock is built so that workloads of mostly readers can be executed with + low runtime overheads. This matches that the default kind of the lock is + PTHREAD_RWLOCK_PREFER_READER_NP. Acquiring a read lock requires a single + atomic addition if the lock is or was previously acquired by other + readers; releasing the lock is a single CAS if there are no concurrent + writers. + Workloads consisting of mostly writers are of secondary importance. + An uncontended write lock acquisition is as fast as for a normal + exclusive mutex but writer contention is somewhat more costly due to + keeping track of the exact number of writers. If the rwlock kind requests + writers to be preferred (i.e., PTHREAD_RWLOCK_PREFER_WRITERS_NP or the + no-recursive-readers variant of it), then writer--to--writer lock ownership + hand-over is fairly fast and bypasses lock acquisition attempts by readers. + The costs of lock ownership transfer between readers and writers vary. If + the program asserts that there are no recursive readers and writers are + preferred, then write lock acquisition attempts will block subsequent read + lock acquisition attempts, so that new incoming readers do not prolong a + phase in which readers have acquired the lock. + + + The main components of the rwlock are a writer-only lock that allows only + one of the concurrent writers to be the primary writer, and a + single-writer-multiple-readers lock that decides between read phases, in + which readers have acquired the rwlock, and write phases in which a primary + writer or a sequence of different primary writers have acquired the rwlock. + + The single-writer-multiple-readers lock is the central piece of state + describing the rwlock and is encoded in the __readers field (see below for + a detailed explanation): + + State WP WL R RW Notes + --------------------------- + #1 0 0 0 0 Lock is idle (and in a read phase). + #2 0 0 >0 0 Readers have acquired the lock. + #3 0 1 0 0 Lock is not acquired; a writer is waiting for a write + phase to start or will try to start one. + #4 0 1 >0 0 Readers have acquired the lock; a writer is waiting + and explicit hand-over to the writer is required. + #4a 0 1 >0 1 Same as #4 except that there are further readers + waiting because the writer is to be preferred. + #5 1 0 0 0 Lock is idle (and in a write phase). + #6 1 0 >0 0 Write phase; readers are waiting for a read phase to + start or will try to start one. + #7 1 1 0 0 Lock is acquired by a writer. + #8 1 1 >0 0 Lock acquired by a writer and readers are waiting; + explicit hand-over to the readers is required. + + WP (PTHREAD_RWLOCK_WRPHASE) is true if the lock is in a write phase, so + potentially acquired by a primary writer. + WL (PTHREAD_RWLOCK_WRLOCKED) is true if there is a primary writer (i.e., + the thread that was able to set this bit from false to true). + R (all bits in __readers except the number of least-significant bits + denoted in PTHREAD_RWLOCK_READER_SHIFT) is the number of readers that have + or are trying to acquired the lock. There may be more readers waiting if + writers are preferred and there will be no recursive readers, in which + case RW (PTHREAD_RWLOCK_RWAITING) is true in state #4a. + + We want to block using futexes but using __readers as a futex word directly + is not a good solution. First, we want to wait on different conditions + such as waiting for a phase change vs. waiting for the primary writer to + release the writer-only lock. Second, the number of readers could change + frequently, which would make it likely that a writer's futex_wait fails + frequently too because the expected value does not match the value of + __readers anymore. + Therefore, we split out the futex words into the __wrphase_futex and + __writers_futex fields. The former tracks the value of the WP bit and is + changed after changing WP by the thread that changes WP. However, because + of the POSIX requirements regarding mutex/rwlock destruction (i.e., that + destroying a rwlock is allowed as soon as no thread has acquired or will + acquire the lock), we have to be careful and hand over lock ownership (via + a phase change) carefully to those threads waiting. Specifically, we must + prevent a situation in which we are not quite sure whether we still have + to unblock another thread through a change to memory (executing a + futex_wake on a former futex word that is now used for something else is + fine). + The scheme we use for __wrphase_futex is that waiting threads that may + use the futex word to block now all have to use the futex word to block; it + is not allowed to take the short-cut and spin-wait on __readers because + then the waking thread cannot just make one final change to memory to + unblock all potentially waiting threads. If, for example, a reader + increments R in states #7 or #8, it has to then block until __wrphase_futex + is 0 and it can confirm that the value of 0 was stored by the primary + writer; in turn, the primary writer has to change to a read phase too when + releasing WL (i.e., to state #2), and it must change __wrphase_futex to 0 + as the next step. This ensures that the waiting reader will not be able to + acquire, release, and then destroy the lock concurrently with the pending + futex unblock operations by the former primary writer. This scheme is + called explicit hand-over in what follows. + Note that waiting threads can cancel waiting only if explicit hand-over has + not yet started (e.g., if __readers is still in states #7 or #8 in the + example above). + + Writers determine the primary writer through WL. Blocking using futexes + is performed using __writers_futex as a futex word; primary writers will + enable waiting on this futex by setting it to 1 after they acquired the WL + bit and will disable waiting by setting it to 0 before they release WL. + This leaves small windows where blocking using futexes is not possible + although a primary writer exists, but in turn decreases complexity of the + writer--writer synchronization and does not affect correctness. + If writers are preferred, writers can hand over WL directly to other + waiting writers that registered by incrementing __writers: If the primary + writer can CAS __writers from a non-zero value to the same value with the + PTHREAD_RWLOCK_WRHANDOVER bit set, it effectively transfers WL ownership + to one of the registered waiting writers and does not reset WL; in turn, + a registered writer that can clear PTHREAD_RWLOCK_WRHANDOVER using a CAS + then takes over WL. Note that registered waiting writers can cancel + waiting by decrementing __writers, but the last writer to unregister must + become the primary writer if PTHREAD_RWLOCK_WRHANDOVER is set. + Also note that adding another state/bit to signal potential writer--writer + contention (e.g., as done in the normal mutex algorithm) would not be + helpful because we would have to conservatively assume that there is in + fact no other writer, and wake up readers too. + + To avoid having to call futex_wake when no thread uses __wrphase_futex or + __writers_futex, threads will set the PTHREAD_RWLOCK_FUTEX_USED bit in the + respective futex words before waiting on it (using a CAS so it will only be + set if in a state in which waiting would be possible). In the case of + __writers_futex, we wake only one thread but several threads may share + PTHREAD_RWLOCK_FUTEX_USED, so we must assume that there are still others. + This is similar to what we do in pthread_mutex_lock. We do not need to + do this for __wrphase_futex because there, we always wake all waiting + threads. + + Blocking in the state #4a simply uses __readers as futex word. This + simplifies the algorithm but suffers from some of the drawbacks discussed + before, though not to the same extent because R can only decrease in this + state, so the number of potentially failing futex_wait attempts will be + bounded. All threads moving from state #4a to another state must wake + up threads blocked on the __readers futex. + + The ordering invariants that we have to take care of in the implementation + are primarily those necessary for a reader--writer lock; this is rather + straightforward and happens during write/read phase switching (potentially + through explicit hand-over), and between writers through synchronization + involving the PTHREAD_RWLOCK_WRLOCKED or PTHREAD_RWLOCK_WRHANDOVER bits. + Additionally, we need to take care that modifications of __writers_futex + and __wrphase_futex (e.g., by otherwise unordered readers) take place in + the writer critical sections or read/write phases, respectively, and that + explicit hand-over observes stores from the previous phase. How this is + done is explained in more detail in comments in the code. + + Many of the accesses to the futex words just need relaxed MO. This is + possible because we essentially drive both the core rwlock synchronization + and the futex synchronization in parallel. For example, an unlock will + unlock the rwlock and take part in the futex synchronization (using + PTHREAD_RWLOCK_FUTEX_USED, see above); even if they are not tightly + ordered in some way, the futex synchronization ensures that there are no + lost wake-ups, and woken threads will then eventually see the most recent + state of the rwlock. IOW, waiting threads will always be woken up, while + not being able to wait using futexes (which can happen) is harmless; in + turn, this means that waiting threads don't need special ordering wrt. + waking threads. + + The futex synchronization consists of the three-state futex word: + (1) cannot block on it, (2) can block on it, and (3) there might be a + thread blocked on it (i.e., with PTHREAD_RWLOCK_FUTEX_USED set). + Relaxed-MO atomic read-modify-write operations are sufficient to maintain + this (e.g., using a CAS to go from (2) to (3) but not from (1) to (3)), + but we need ordering of the futex word modifications by the waking threads + so that they collectively make correct state changes between (1)-(3). + The futex-internal synchronization (i.e., the conceptual critical sections + around futex operations in the kernel) then ensures that even an + unconstrained load (i.e., relaxed MO) inside of futex_wait will not lead to + lost wake-ups because either the waiting thread will see the change from + (3) to (1) when a futex_wake came first, or this futex_wake will wake this + waiting thread because the waiting thread came first. + + + POSIX allows but does not require rwlock acquisitions to be a cancellation + point. We do not support cancellation. + + TODO We do not try to elide any read or write lock acquisitions currently. + While this would be possible, it is unclear whether HTM performance is + currently predictable enough and our runtime tuning is good enough at + deciding when to use elision so that enabling it would lead to consistently + better performance. */ + + +static int +__pthread_rwlock_get_private (pthread_rwlock_t *rwlock) +{ + return rwlock->__data.__shared != 0 ? FUTEX_SHARED : FUTEX_PRIVATE; +} + +static __always_inline void +__pthread_rwlock_rdunlock (pthread_rwlock_t *rwlock) +{ + int private = __pthread_rwlock_get_private (rwlock); + /* We decrease the number of readers, and if we are the last reader and + there is a primary writer, we start a write phase. We use a CAS to + make this atomic so that it is clear whether we must hand over ownership + explicitly. */ + unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); + unsigned int rnew; + for (;;) + { + rnew = r - (1 << PTHREAD_RWLOCK_READER_SHIFT); + /* If we are the last reader, we also need to unblock any readers + that are waiting for a writer to go first (PTHREAD_RWLOCK_RWAITING) + so that they can register while the writer is active. */ + if ((rnew >> PTHREAD_RWLOCK_READER_SHIFT) == 0) + { + if ((rnew & PTHREAD_RWLOCK_WRLOCKED) != 0) + rnew |= PTHREAD_RWLOCK_WRPHASE; + rnew &= ~(unsigned int) PTHREAD_RWLOCK_RWAITING; + } + /* We need release MO here for three reasons. First, so that we + synchronize with subsequent writers. Second, we might have been the + first reader and set __wrphase_futex to 0, so we need to synchronize + with the last reader that will set it to 1 (note that we will always + change __readers before the last reader, or we are the last reader). + Third, a writer that takes part in explicit hand-over needs to see + the first reader's store to __wrphase_futex (or a later value) if + the writer observes that a write phase has been started. */ + if (atomic_compare_exchange_weak_release (&rwlock->__data.__readers, + &r, rnew)) + break; + /* TODO Back-off. */ + } + if ((rnew & PTHREAD_RWLOCK_WRPHASE) != 0) + { + /* We need to do explicit hand-over. We need the acquire MO fence so + that our modification of _wrphase_futex happens after a store by + another reader that started a read phase. Relaxed MO is sufficient + for the modification of __wrphase_futex because it is just used + to delay acquisition by a writer until all threads are unblocked + irrespective of whether they are looking at __readers or + __wrphase_futex; any other synchronizes-with relations that are + necessary are established through __readers. */ + atomic_thread_fence_acquire (); + if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 1) + & PTHREAD_RWLOCK_FUTEX_USED) != 0) + futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private); + } + /* Also wake up waiting readers if we did reset the RWAITING flag. */ + if ((r & PTHREAD_RWLOCK_RWAITING) != (rnew & PTHREAD_RWLOCK_RWAITING)) + futex_wake (&rwlock->__data.__readers, INT_MAX, private); +} + + +static __always_inline int +__pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock, + const struct timespec *abstime) +{ + unsigned int r; + + /* Make sure we are not holding the rwlock as a writer. This is a deadlock + situation we recognize and report. */ + if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer) + == THREAD_GETMEM (THREAD_SELF, tid))) + return EDEADLK; + + /* If we prefer writers, recursive rdlock is disallowed, we are in a read + phase, and there are other readers present, we try to wait without + extending the read phase. We will be unblocked by either one of the + other active readers, or if the writer gives up WRLOCKED (e.g., on + timeout). + If there are no other readers, we simply race with any existing primary + writer; it would have been a race anyway, and changing the odds slightly + will likely not make a big difference. */ + if (rwlock->__data.__flags == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP) + { + r = atomic_load_relaxed (&rwlock->__data.__readers); + while (((r & PTHREAD_RWLOCK_WRPHASE) == 0) + && ((r & PTHREAD_RWLOCK_WRLOCKED) != 0) + && ((r >> PTHREAD_RWLOCK_READER_SHIFT) > 0)) + { + /* TODO Spin first. */ + /* Try setting the flag signaling that we are waiting without having + incremented the number of readers. Relaxed MO is fine because + this is just about waiting for a state change in __readers. */ + if (atomic_compare_exchange_weak_relaxed + (&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_RWAITING)) + { + /* Wait for as long as the flag is set. An ABA situation is + harmless because the flag is just about the state of + __readers, and all threads set the flag under the same + conditions. */ + while ((atomic_load_relaxed (&rwlock->__data.__readers) + & PTHREAD_RWLOCK_RWAITING) != 0) + { + int private = __pthread_rwlock_get_private (rwlock); + int err = futex_abstimed_wait (&rwlock->__data.__readers, + r, abstime, private); + /* We ignore EAGAIN and EINTR. On time-outs, we can just + return because we don't need to clean up anything. */ + if (err == ETIMEDOUT) + return err; + } + /* It makes sense to not break out of the outer loop here + because we might be in the same situation again. */ + } + else + { + /* TODO Back-off. */ + } + } + } + /* Register as a reader, using an add-and-fetch so that R can be used as + expected value for future operations. Acquire MO so we synchronize with + prior writers as well as the last reader of the previous read phase (see + below). */ + r = atomic_fetch_add_acquire (&rwlock->__data.__readers, + (1 << PTHREAD_RWLOCK_READER_SHIFT)) + (1 << PTHREAD_RWLOCK_READER_SHIFT); + + /* Check whether there is an overflow in the number of readers. We assume + that the total number of threads is less than half the maximum number + of readers that we have bits for in __readers (i.e., with 32-bit int and + PTHREAD_RWLOCK_READER_SHIFT of 3, we assume there are less than + 1 << (32-3-1) concurrent threads). + If there is an overflow, we use a CAS to try to decrement the number of + readers if there still is an overflow situation. If so, we return + EAGAIN; if not, we are not a thread causing an overflow situation, and so + we just continue. Using a fetch-add instead of the CAS isn't possible + because other readers might release the lock concurrently, which could + make us the last reader and thus responsible for handing ownership over + to writers (which requires a CAS too to make the decrement and ownership + transfer indivisible). */ + while (__glibc_unlikely (r >= PTHREAD_RWLOCK_READER_OVERFLOW)) + { + /* Relaxed MO is okay because we just want to undo our registration and + cannot have changed the rwlock state substantially if the CAS + succeeds. */ + if (atomic_compare_exchange_weak_relaxed (&rwlock->__data.__readers, &r, + r - (1 << PTHREAD_RWLOCK_READER_SHIFT))) + return EAGAIN; + } + + /* We have registered as a reader, so if we are in a read phase, we have + acquired a read lock. This is also the reader--reader fast-path. + Even if there is a primary writer, we just return. If writers are to + be preferred and we are the only active reader, we could try to enter a + write phase to let the writer proceed. This would be okay because we + cannot have acquired the lock previously as a reader (which could result + in deadlock if we would wait for the primary writer to run). However, + this seems to be a corner case and handling it specially not be worth the + complexity. */ + if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0)) + return 0; + + /* If there is no primary writer but we are in a write phase, we can try + to install a read phase ourself. */ + while (((r & PTHREAD_RWLOCK_WRPHASE) != 0) + && ((r & PTHREAD_RWLOCK_WRLOCKED) == 0)) + { + /* Try to enter a read phase: If the CAS below succeeds, we have + ownership; if it fails, we will simply retry and reassess the + situation. + Acquire MO so we synchronize with prior writers. */ + if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r, + r ^ PTHREAD_RWLOCK_WRPHASE)) + { + /* We started the read phase, so we are also responsible for + updating the write-phase futex. Relaxed MO is sufficient. + Note that there can be no other reader that we have to wake + because all other readers will see the read phase started by us + (or they will try to start it themselves); if a writer started + the read phase, we cannot have started it. Furthermore, we + cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will + overwrite the value set by the most recent writer (or the readers + before it in case of explicit hand-over) and we know that there + are no waiting readers. */ + atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0); + return 0; + } + else + { + /* TODO Back off before retrying. Also see above. */ + } + } + + if ((r & PTHREAD_RWLOCK_WRPHASE) != 0) + { + /* We are in a write phase, and there must be a primary writer because + of the previous loop. Block until the primary writer gives up the + write phase. This case requires explicit hand-over using + __wrphase_futex. + However, __wrphase_futex might not have been set to 1 yet (either + because explicit hand-over to the writer is still ongoing, or because + the writer has started the write phase but does not yet have updated + __wrphase_futex). The least recent value of __wrphase_futex we can + read from here is the modification of the last read phase (because + we synchronize with the last reader in this read phase through + __readers; see the use of acquire MO on the fetch_add above). + Therefore, if we observe a value of 0 for __wrphase_futex, we need + to subsequently check that __readers now indicates a read phase; we + need to use acquire MO for this so that if we observe a read phase, + we will also see the modification of __wrphase_futex by the previous + writer. We then need to load __wrphase_futex again and continue to + wait if it is not 0, so that we do not skip explicit hand-over. + Relaxed MO is sufficient for the load from __wrphase_futex because + we just use it as an indicator for when we can proceed; we use + __readers and the acquire MO accesses to it to eventually read from + the proper stores to __wrphase_futex. */ + unsigned int wpf; + bool ready = false; + for (;;) + { + while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex)) + | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED)) + { + int private = __pthread_rwlock_get_private (rwlock); + if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0) + && !atomic_compare_exchange_weak_relaxed + (&rwlock->__data.__wrphase_futex, + &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED)) + continue; + int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex, + 1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private); + if (err == ETIMEDOUT) + { + /* If we timed out, we need to unregister. If no read phase + has been installed while we waited, we can just decrement + the number of readers. Otherwise, we just acquire the + lock, which is allowed because we give no precise timing + guarantees, and because the timeout is only required to + be in effect if we would have had to wait for other + threads (e.g., if futex_wait would time-out immediately + because the given absolute time is in the past). */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + while ((r & PTHREAD_RWLOCK_WRPHASE) != 0) + { + /* We don't need to make anything else visible to + others besides unregistering, so relaxed MO is + sufficient. */ + if (atomic_compare_exchange_weak_relaxed + (&rwlock->__data.__readers, &r, + r - (1 << PTHREAD_RWLOCK_READER_SHIFT))) + return ETIMEDOUT; + /* TODO Back-off. */ + } + /* Use the acquire MO fence to mirror the steps taken in the + non-timeout case. Note that the read can happen both + in the atomic_load above as well as in the failure case + of the CAS operation. */ + atomic_thread_fence_acquire (); + /* We still need to wait for explicit hand-over, but we must + not use futex_wait anymore because we would just time out + in this case and thus make the spin-waiting we need + unnecessarily expensive. */ + while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex) + | PTHREAD_RWLOCK_FUTEX_USED) + == (1 | PTHREAD_RWLOCK_FUTEX_USED)) + { + /* TODO Back-off? */ + } + ready = true; + break; + } + /* If we got interrupted (EINTR) or the futex word does not have the + expected value (EAGAIN), retry. */ + } + if (ready) + /* See below. */ + break; + /* We need acquire MO here so that we synchronize with the lock + release of the writer, and so that we observe a recent value of + __wrphase_futex (see below). */ + if ((atomic_load_acquire (&rwlock->__data.__readers) + & PTHREAD_RWLOCK_WRPHASE) == 0) + /* We are in a read phase now, so the least recent modification of + __wrphase_futex we can read from is the store by the writer + with value 1. Thus, only now we can assume that if we observe + a value of 0, explicit hand-over is finished. Retry the loop + above one more time. */ + ready = true; + } + } + + return 0; +} + + +static __always_inline void +__pthread_rwlock_wrunlock (pthread_rwlock_t *rwlock) +{ + int private = __pthread_rwlock_get_private (rwlock); + + atomic_store_relaxed (&rwlock->__data.__cur_writer, 0); + /* Disable waiting by writers. We will wake up after we decided how to + proceed. */ + bool wake_writers = ((atomic_exchange_relaxed + (&rwlock->__data.__writers_futex, 0) & PTHREAD_RWLOCK_FUTEX_USED) != 0); + + if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP) + { + /* First, try to hand over to another writer. */ + unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers); + while (w != 0) + { + /* Release MO so that another writer that gets WRLOCKED from us will + synchronize with us and thus can take over our view of + __readers (including, for example, whether we are in a write + phase or not). */ + if (atomic_compare_exchange_weak_release (&rwlock->__data.__writers, + &w, w | PTHREAD_RWLOCK_WRHANDOVER)) + /* Another writer will take over. */ + goto done; + /* TODO Back-off. */ + } + } + + /* We have done everything we needed to do to prefer writers, so now we + either hand over explicitly to readers if there are any, or we simply + stay in a write phase. See pthread_rwlock_rdunlock for more details. */ + unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); + /* Release MO so that subsequent readers or writers synchronize with us. */ + while (!atomic_compare_exchange_weak_release + (&rwlock->__data.__readers, &r, (r ^ PTHREAD_RWLOCK_WRLOCKED) + ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0 + : PTHREAD_RWLOCK_WRPHASE))) + { + /* TODO Back-off. */ + } + if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0) + { + /* We must hand over explicitly through __wrphase_futex. Relaxed MO is + sufficient because it is just used to delay acquisition by a writer; + any other synchronizes-with relations that are necessary are + established through __readers. */ + if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0) + & PTHREAD_RWLOCK_FUTEX_USED) != 0) + futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private); + } + + done: + /* We released WRLOCKED in some way, so wake a writer. */ + if (wake_writers) + futex_wake (&rwlock->__data.__writers_futex, 1, private); +} + + +static __always_inline int +__pthread_rwlock_wrlock_full (pthread_rwlock_t *rwlock, + const struct timespec *abstime) +{ + /* Make sure we are not holding the rwlock as a writer. This is a deadlock + situation we recognize and report. */ + if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer) + == THREAD_GETMEM (THREAD_SELF, tid))) + return EDEADLK; + + /* First we try to acquire the role of primary writer by setting WRLOCKED; + if it was set before, there already is a primary writer. Acquire MO so + that we synchronize with previous primary writers. + + We do not try to change to a write phase right away using a fetch_or + because we would have to reset it again and wake readers if there are + readers present (some readers could try to acquire the lock more than + once, so setting a write phase in the middle of this could cause + deadlock). Changing to a write phase eagerly would only speed up the + transition from a read phase to a write phase in the uncontended case, + but it would slow down the contended case if readers are preferred (which + is the default). + We could try to CAS from a state with no readers to a write phase, but + this could be less scalable if readers arrive and leave frequently. */ + bool may_share_futex_used_flag = false; + unsigned int r = atomic_fetch_or_acquire (&rwlock->__data.__readers, + PTHREAD_RWLOCK_WRLOCKED); + if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)) + { + /* There is another primary writer. */ + bool prefer_writer = + (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP); + if (prefer_writer) + { + /* We register as a waiting writer, so that we can make use of + writer--writer hand-over. Relaxed MO is fine because we just + want to register. We assume that the maximum number of threads + is less than the capacity in __writers. */ + atomic_fetch_add_relaxed (&rwlock->__data.__writers, 1); + } + for (;;) + { + /* TODO Spin until WRLOCKED is 0 before trying the CAS below. + But pay attention to not delay trying writer--writer hand-over + for too long (which we must try eventually anyway). */ + if ((r & PTHREAD_RWLOCK_WRLOCKED) == 0) + { + /* Try to become the primary writer or retry. Acquire MO as in + the fetch_or above. */ + if (atomic_compare_exchange_weak_acquire + (&rwlock->__data.__readers, &r, + r | PTHREAD_RWLOCK_WRLOCKED)) + { + if (prefer_writer) + { + /* Unregister as a waiting writer. Note that because we + acquired WRLOCKED, WRHANDOVER will not be set. + Acquire MO on the CAS above ensures that + unregistering happens after the previous writer; + this sorts the accesses to __writers by all + primary writers in a useful way (e.g., any other + primary writer acquiring after us or getting it from + us through WRHANDOVER will see both our changes to + __writers). + ??? Perhaps this is not strictly necessary for + reasons we do not yet know of. */ + atomic_fetch_add_relaxed (&rwlock->__data.__writers, + -1); + } + break; + } + /* Retry if the CAS fails (r will have been updated). */ + continue; + } + /* If writer--writer hand-over is available, try to become the + primary writer this way by grabbing the WRHANDOVER token. If we + succeed, we own WRLOCKED. */ + if (prefer_writer) + { + unsigned int w = atomic_load_relaxed + (&rwlock->__data.__writers); + if ((w & PTHREAD_RWLOCK_WRHANDOVER) != 0) + { + /* Acquire MO is required here so that we synchronize with + the writer that handed over WRLOCKED. We also need this + for the reload of __readers below because our view of + __readers must be at least as recent as the view of the + writer that handed over WRLOCKED; we must avoid an ABA + through WRHANDOVER, which could, for example, lead to us + assuming we are still in a write phase when in fact we + are not. */ + if (atomic_compare_exchange_weak_acquire + (&rwlock->__data.__writers, + &w, (w - PTHREAD_RWLOCK_WRHANDOVER - 1))) + { + /* Reload so our view is consistent with the view of + the previous owner of WRLOCKED. See above. */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + break; + } + /* We do not need to reload __readers here. We should try + to perform writer--writer hand-over if possible; if it + is not possible anymore, we will reload __readers + elsewhere in this loop. */ + continue; + } + } + /* We did not acquire WRLOCKED nor were able to use writer--writer + hand-over, so we block on __writers_futex. */ + int private = __pthread_rwlock_get_private (rwlock); + unsigned int wf = atomic_load_relaxed + (&rwlock->__data.__writers_futex); + if (((wf & ~(unsigned int) PTHREAD_RWLOCK_FUTEX_USED) != 1) + || ((wf != (1 | PTHREAD_RWLOCK_FUTEX_USED)) + && !atomic_compare_exchange_weak_relaxed + (&rwlock->__data.__writers_futex, &wf, + 1 | PTHREAD_RWLOCK_FUTEX_USED))) + { + /* If we cannot block on __writers_futex because there is no + primary writer, or we cannot set PTHREAD_RWLOCK_FUTEX_USED, + we retry. We must reload __readers here in case we cannot + block on __writers_futex so that we can become the primary + writer and are not stuck in a loop that just continuously + fails to block on __writers_futex. */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + continue; + } + /* We set the flag that signals that the futex is used, or we could + have set it if we had been faster than other waiters. As a + result, we may share the flag with an unknown number of other + writers. Therefore, we must keep this flag set when we acquire + the lock. We do not need to do this when we do not reach this + point here because then we are not part of the group that may + share the flag, and another writer will wake one of the writers + in this group. */ + may_share_futex_used_flag = true; + int err = futex_abstimed_wait (&rwlock->__data.__writers_futex, + 1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private); + if (err == ETIMEDOUT) + { + if (prefer_writer) + { + /* We need to unregister as a waiting writer. If we are the + last writer and writer--writer hand-over is available, + we must make use of it because nobody else will reset + WRLOCKED otherwise. (If we use it, we simply pretend + that this happened before the timeout; see + pthread_rwlock_rdlock_full for the full reasoning.) + Also see the similar code above. */ + unsigned int w = atomic_load_relaxed + (&rwlock->__data.__writers); + while (!atomic_compare_exchange_weak_acquire + (&rwlock->__data.__writers, &w, + (w == PTHREAD_RWLOCK_WRHANDOVER + 1 ? 0 : w - 1))) + { + /* TODO Back-off. */ + } + if (w == PTHREAD_RWLOCK_WRHANDOVER + 1) + { + /* We must continue as primary writer. See above. */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + break; + } + } + /* We cleaned up and cannot have stolen another waiting writer's + futex wake-up, so just return. */ + return ETIMEDOUT; + } + /* If we got interrupted (EINTR) or the futex word does not have the + expected value (EAGAIN), retry after reloading __readers. */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + } + /* Our snapshot of __readers is up-to-date at this point because we + either set WRLOCKED using a CAS or were handed over WRLOCKED from + another writer whose snapshot of __readers we inherit. */ + } + + /* If we are in a read phase and there are no readers, try to start a write + phase. */ + while (((r & PTHREAD_RWLOCK_WRPHASE) == 0) + && ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)) + { + /* Acquire MO so that we synchronize with prior writers and do + not interfere with their updates to __writers_futex, as well + as regarding prior readers and their updates to __wrphase_futex, + respectively. */ + if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, + &r, r | PTHREAD_RWLOCK_WRPHASE)) + { + /* We have started a write phase, so need to enable readers to wait. + See the similar case in__pthread_rwlock_rdlock_full. */ + atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); + /* Make sure we fall through to the end of the function. */ + r |= PTHREAD_RWLOCK_WRPHASE; + break; + } + /* TODO Back-off. */ + } + + /* We are the primary writer; enable blocking on __writers_futex. Relaxed + MO is sufficient for futex words; acquire MO on the previous + modifications of __readers ensures that this store happens after the + store of value 0 by the previous primary writer. */ + atomic_store_relaxed (&rwlock->__data.__writers_futex, + 1 | (may_share_futex_used_flag ? PTHREAD_RWLOCK_FUTEX_USED : 0)); + + if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRPHASE) == 0)) + { + /* We are not in a read phase and there are readers (because of the + previous loop). Thus, we have to wait for explicit hand-over from + one of these readers. + We basically do the same steps as for the similar case in + __pthread_rwlock_rdlock_full, except that we additionally might try + to directly hand over to another writer and need to wake up + other writers or waiting readers (i.e., PTHREAD_RWLOCK_RWAITING). */ + unsigned int wpf; + bool ready = false; + for (;;) + { + while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex)) + | PTHREAD_RWLOCK_FUTEX_USED) == PTHREAD_RWLOCK_FUTEX_USED) + { + int private = __pthread_rwlock_get_private (rwlock); + if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0) + && !atomic_compare_exchange_weak_relaxed + (&rwlock->__data.__wrphase_futex, &wpf, + PTHREAD_RWLOCK_FUTEX_USED)) + continue; + int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex, + PTHREAD_RWLOCK_FUTEX_USED, abstime, private); + if (err == ETIMEDOUT) + { + if (rwlock->__data.__flags + != PTHREAD_RWLOCK_PREFER_READER_NP) + { + /* We try writer--writer hand-over. */ + unsigned int w = atomic_load_relaxed + (&rwlock->__data.__writers); + if (w != 0) + { + /* We are about to hand over WRLOCKED, so we must + release __writers_futex too; otherwise, we'd have + a pending store, which could at least prevent + other threads from waiting using the futex + because it could interleave with the stores + by subsequent writers. In turn, this means that + we have to clean up when we do not hand over + WRLOCKED. + Release MO so that another writer that gets + WRLOCKED from us can take over our view of + __readers. */ + unsigned int wf = atomic_exchange_relaxed + (&rwlock->__data.__writers_futex, 0); + while (w != 0) + { + if (atomic_compare_exchange_weak_release + (&rwlock->__data.__writers, &w, + w | PTHREAD_RWLOCK_WRHANDOVER)) + { + /* Wake other writers. */ + if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0) + futex_wake + (&rwlock->__data.__writers_futex, 1, + private); + return ETIMEDOUT; + } + /* TODO Back-off. */ + } + /* We still own WRLOCKED and someone else might set + a write phase concurrently, so enable waiting + again. Make sure we don't loose the flag that + signals whether there are threads waiting on + this futex. */ + atomic_store_relaxed + (&rwlock->__data.__writers_futex, wf); + } + } + /* If we timed out and we are not in a write phase, we can + just stop being a primary writer. Otherwise, we just + acquire the lock. */ + r = atomic_load_relaxed (&rwlock->__data.__readers); + if ((r & PTHREAD_RWLOCK_WRPHASE) == 0) + { + /* We are about to release WRLOCKED, so we must release + __writers_futex too; see the handling of + writer--writer hand-over above. */ + unsigned int wf = atomic_exchange_relaxed + (&rwlock->__data.__writers_futex, 0); + while ((r & PTHREAD_RWLOCK_WRPHASE) == 0) + { + /* While we don't need to make anything from a + caller's critical section visible to other + threads, we need to ensure that our changes to + __writers_futex are properly ordered. + Therefore, use release MO to synchronize with + subsequent primary writers. Also wake up any + waiting readers as they are waiting because of + us. */ + if (atomic_compare_exchange_weak_release + (&rwlock->__data.__readers, &r, + (r ^ PTHREAD_RWLOCK_WRLOCKED) + & ~(unsigned int) PTHREAD_RWLOCK_RWAITING)) + { + /* Wake other writers. */ + if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0) + futex_wake (&rwlock->__data.__writers_futex, + 1, private); + /* Wake waiting readers. */ + if ((r & PTHREAD_RWLOCK_RWAITING) != 0) + futex_wake (&rwlock->__data.__readers, + INT_MAX, private); + return ETIMEDOUT; + } + } + /* We still own WRLOCKED and someone else might set a + write phase concurrently, so enable waiting again. + Make sure we don't loose the flag that signals + whether there are threads waiting on this futex. */ + atomic_store_relaxed (&rwlock->__data.__writers_futex, + wf); + } + /* Use the acquire MO fence to mirror the steps taken in the + non-timeout case. Note that the read can happen both + in the atomic_load above as well as in the failure case + of the CAS operation. */ + atomic_thread_fence_acquire (); + /* We still need to wait for explicit hand-over, but we must + not use futex_wait anymore. */ + while ((atomic_load_relaxed + (&rwlock->__data.__wrphase_futex) + | PTHREAD_RWLOCK_FUTEX_USED) + == PTHREAD_RWLOCK_FUTEX_USED) + { + /* TODO Back-off. */ + } + ready = true; + break; + } + /* If we got interrupted (EINTR) or the futex word does not have + the expected value (EAGAIN), retry. */ + } + /* See pthread_rwlock_rdlock_full. */ + if (ready) + break; + if ((atomic_load_acquire (&rwlock->__data.__readers) + & PTHREAD_RWLOCK_WRPHASE) != 0) + ready = true; + } + } + + atomic_store_relaxed (&rwlock->__data.__cur_writer, + THREAD_GETMEM (THREAD_SELF, tid)); + return 0; +} |