diff options
Diffstat (limited to 'sysdeps/pthread/tst-rwlock-tryrdlock-stall.c')
| -rw-r--r-- | sysdeps/pthread/tst-rwlock-tryrdlock-stall.c | 355 |
1 files changed, 355 insertions, 0 deletions
diff --git a/sysdeps/pthread/tst-rwlock-tryrdlock-stall.c b/sysdeps/pthread/tst-rwlock-tryrdlock-stall.c new file mode 100644 index 0000000000..bb2a64f301 --- /dev/null +++ b/sysdeps/pthread/tst-rwlock-tryrdlock-stall.c @@ -0,0 +1,355 @@ +/* Bug 23844: Test for pthread_rwlock_tryrdlock stalls. + Copyright (C) 2019-2020 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 + <https://www.gnu.org/licenses/>. */ + +/* For a full analysis see comment: + https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14 + + Provided here for reference: + + --- Analysis of pthread_rwlock_tryrdlock() stall --- + A read lock begins to execute. + + In __pthread_rwlock_rdlock_full: + + We can attempt a read lock, but find that the lock is + in a write phase (PTHREAD_RWLOCK_WRPHASE, or WP-bit + is set), and the lock is held by a primary writer + (PTHREAD_RWLOCK_WRLOCKED is set). In this case we must + wait for explicit hand over from the writer to us or + one of the other waiters. The read lock threads are + about to execute: + + 341 r = (atomic_fetch_add_acquire (&rwlock->__data.__readers, + 342 (1 << PTHREAD_RWLOCK_READER_SHIFT)) + 343 + (1 << PTHREAD_RWLOCK_READER_SHIFT)); + + An unlock beings to execute. + + Then in __pthread_rwlock_wrunlock: + + 547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); + ... + 549 while (!atomic_compare_exchange_weak_release + 550 (&rwlock->__data.__readers, &r, + 551 ((r ^ PTHREAD_RWLOCK_WRLOCKED) + 552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0 + 553 : PTHREAD_RWLOCK_WRPHASE)))) + 554 { + ... + 556 } + + We clear PTHREAD_RWLOCK_WRLOCKED, and if there are + no readers so we leave the lock in PTHRAD_RWLOCK_WRPHASE. + + Back in the read lock. + + The read lock adjusts __readres as above. + + 383 while ((r & PTHREAD_RWLOCK_WRPHASE) != 0 + 384 && (r & PTHREAD_RWLOCK_WRLOCKED) == 0) + 385 { + ... + 390 if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r, + 391 r ^ PTHREAD_RWLOCK_WRPHASE)) + 392 { + + And then attemps to start the read phase. + + Assume there happens to be a tryrdlock at this point, noting + that PTHREAD_RWLOCK_WRLOCKED is clear, and PTHREAD_RWLOCK_WRPHASE + is 1. So the try lock attemps to start the read phase. + + In __pthread_rwlock_tryrdlock: + + 44 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0) + 45 { + ... + 49 if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0) + 50 && (rwlock->__data.__flags + 51 == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)) + 52 return EBUSY; + 53 rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT); + 54 } + ... + 89 while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, + 90 &r, rnew)); + + And succeeds. + + Back in the write unlock: + + 557 if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0) + 558 { + ... + 563 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0) + 564 & PTHREAD_RWLOCK_FUTEX_USED) != 0) + 565 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private); + 566 } + + We note that PTHREAD_RWLOCK_FUTEX_USED is non-zero + and don't wake anyone. This is OK because we handed + over to the trylock. It will be the trylock's responsibility + to wake any waiters. + + Back in the read lock: + + The read lock fails to install PTHRAD_REWLOCK_WRPHASE as 0 because + the __readers value was adjusted by the trylock, and so it falls through + to waiting on the lock for explicit handover from either a new writer + or a new reader. + + 448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex, + 449 1 | PTHREAD_RWLOCK_FUTEX_USED, + 450 abstime, private); + + We use PTHREAD_RWLOCK_FUTEX_USED to indicate the futex + is in use. + + At this point we have readers waiting on the read lock + to unlock. The wrlock is done. The trylock is finishing + the installation of the read phase. + + 92 if ((r & PTHREAD_RWLOCK_WRPHASE) != 0) + 93 { + ... + 105 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0); + 106 } + + The trylock does note that we were the one that + installed the read phase, but the comments are not + correct, the execution ordering above shows that + readers might indeed be waiting, and they are. + + The atomic_store_relaxed throws away PTHREAD_RWLOCK_FUTEX_USED, + and the waiting reader is never worken becuase as noted + above it is conditional on the futex being used. + + The solution is for the trylock thread to inspect + PTHREAD_RWLOCK_FUTEX_USED and wake the waiting readers. + + --- Analysis of pthread_rwlock_trywrlock() stall --- + + A write lock begins to execute, takes the write lock, + and then releases the lock... + + In pthread_rwlock_wrunlock(): + + 547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); + ... + 549 while (!atomic_compare_exchange_weak_release + 550 (&rwlock->__data.__readers, &r, + 551 ((r ^ PTHREAD_RWLOCK_WRLOCKED) + 552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0 + 553 : PTHREAD_RWLOCK_WRPHASE)))) + 554 { + ... + 556 } + + ... leaving it in the write phase with zero readers + (the case where we leave the write phase in place + during a write unlock). + + A write trylock begins to execute. + + In __pthread_rwlock_trywrlock: + + 40 while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0) + 41 && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0) + 42 || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0)))) + 43 { + + The lock is not locked. + + There are no readers. + + 45 if (atomic_compare_exchange_weak_acquire ( + 46 &rwlock->__data.__readers, &r, + 47 r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED)) + + We atomically install the write phase and we take the + exclusive write lock. + + 48 { + 49 atomic_store_relaxed (&rwlock->__data.__writers_futex, 1); + + We get this far. + + A reader lock begins to execute. + + In pthread_rwlock_rdlock: + + 437 for (;;) + 438 { + 439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex)) + 440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED)) + 441 { + 442 int private = __pthread_rwlock_get_private (rwlock); + 443 if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0) + 444 && (!atomic_compare_exchange_weak_relaxed + 445 (&rwlock->__data.__wrphase_futex, + 446 &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED))) + 447 continue; + 448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex, + 449 1 | PTHREAD_RWLOCK_FUTEX_USED, + 450 abstime, private); + + We are in a write phase, so the while() on line 439 is true. + + The value of wpf does not have PTHREAD_RWLOCK_FUTEX_USED set + since this is the first reader to lock. + + The atomic operation sets wpf with PTHREAD_RELOCK_FUTEX_USED + on the expectation that this reader will be woken during + the handoff. + + Back in pthread_rwlock_trywrlock: + + 50 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); + 51 atomic_store_relaxed (&rwlock->__data.__cur_writer, + 52 THREAD_GETMEM (THREAD_SELF, tid)); + 53 return 0; + 54 } + ... + 57 } + + We write 1 to __wrphase_futex discarding PTHREAD_RWLOCK_FUTEX_USED, + and so in the unlock we will not awaken the waiting reader. + + The solution to this is to realize that if we did not start the write + phase we need not write 1 or any other value to __wrphase_futex. + This ensures that any readers (which saw __wrphase_futex != 0) can + set PTHREAD_RWLOCK_FUTEX_USED and this can be used at unlock to + wake them. + + If we installed the write phase then all other readers are looping + here: + + In __pthread_rwlock_rdlock_full: + + 437 for (;;) + 438 { + 439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex)) + 440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED)) + 441 { + ... + 508 } + + waiting for the write phase to be installed or removed before they + can begin waiting on __wrphase_futex (part of the algorithm), or + taking a concurrent read lock, and thus we can safely write 1 to + __wrphase_futex. + + If we did not install the write phase then the readers may already + be waiting on the futex, the original writer wrote 1 to __wrphase_futex + as part of starting the write phase, and we cannot also write 1 + without loosing the PTHREAD_RWLOCK_FUTEX_USED bit. + + --- + + Summary for the pthread_rwlock_tryrdlock() stall: + + The stall is caused by pthread_rwlock_tryrdlock failing to check + that PTHREAD_RWLOCK_FUTEX_USED is set in the __wrphase_futex futex + and then waking the futex. + + The fix for bug 23844 ensures that waiters on __wrphase_futex are + correctly woken. Before the fix the test stalls as readers can + wait forever on __wrphase_futex. */ + +#include <stdio.h> +#include <stdlib.h> +#include <unistd.h> +#include <pthread.h> +#include <support/xthread.h> +#include <errno.h> + +/* We need only one lock to reproduce the issue. We will need multiple + threads to get the exact case where we have a read, try, and unlock + all interleaving to produce the case where the readers are waiting + and the try fails to wake them. */ +pthread_rwlock_t onelock; + +/* The number of threads is arbitrary but empirically chosen to have + enough threads that we see the condition where waiting readers are + not woken by a successful tryrdlock. */ +#define NTHREADS 32 + +_Atomic int do_exit; + +void * +run_loop (void *arg) +{ + int i = 0, ret; + while (!do_exit) + { + /* Arbitrarily choose if we are the writer or reader. Choose a + high enough ratio of readers to writers to make it likely + that readers block (and eventually are susceptable to + stalling). + + If we are a writer, take the write lock, and then unlock. + If we are a reader, try the lock, then lock, then unlock. */ + if ((i % 8) != 0) + xpthread_rwlock_wrlock (&onelock); + else + { + if ((ret = pthread_rwlock_tryrdlock (&onelock)) != 0) + { + if (ret == EBUSY) + xpthread_rwlock_rdlock (&onelock); + else + exit (EXIT_FAILURE); + } + } + /* Thread does some work and then unlocks. */ + xpthread_rwlock_unlock (&onelock); + i++; + } + return NULL; +} + +int +do_test (void) +{ + int i; + pthread_t tids[NTHREADS]; + xpthread_rwlock_init (&onelock, NULL); + for (i = 0; i < NTHREADS; i++) + tids[i] = xpthread_create (NULL, run_loop, NULL); + /* Run for some amount of time. Empirically speaking exercising + the stall via pthread_rwlock_tryrdlock is much harder, and on + a 3.5GHz 4 core x86_64 VM system it takes somewhere around + 20-200s to stall, approaching 100% stall past 200s. We can't + wait that long for a regression test so we just test for 20s, + and expect the stall to happen with a 5-10% chance (enough for + developers to see). */ + sleep (20); + /* Then exit. */ + printf ("INFO: Exiting...\n"); + do_exit = 1; + /* If any readers stalled then we will timeout waiting for them. */ + for (i = 0; i < NTHREADS; i++) + xpthread_join (tids[i]); + printf ("INFO: Done.\n"); + xpthread_rwlock_destroy (&onelock); + printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n"); + return 0; +} + +#define TIMEOUT 30 +#include <support/test-driver.c> |