From 65810f0ef05e8c9e333f17a44e77808b163ca298 Mon Sep 17 00:00:00 2001 From: Torvald Riegel Date: Thu, 22 Dec 2016 10:20:43 +0100 Subject: robust mutexes: Fix broken x86 assembly by removing it lll_robust_unlock on i386 and x86_64 first sets the futex word to FUTEX_WAITERS|0 before calling __lll_unlock_wake, which will set the futex word to 0. If the thread is killed between these steps, then the futex word will be FUTEX_WAITERS|0, and the kernel (at least current upstream) will not set it to FUTEX_OWNER_DIED|FUTEX_WAITERS because 0 is not equal to the TID of the crashed thread. The lll_robust_lock assembly code on i386 and x86_64 is not prepared to deal with this case because the fastpath tries to only CAS 0 to TID and not FUTEX_WAITERS|0 to TID; the slowpath simply waits until it can CAS 0 to TID or the futex_word has the FUTEX_OWNER_DIED bit set. This issue is fixed by removing the custom x86 assembly code and using the generic C code instead. However, instead of adding more duplicate code to the custom x86 lowlevellock.h, the code of the lll_robust* functions is inlined into the single call sites that exist for each of these functions in the pthread_mutex_* functions. The robust mutex paths in the latter have been slightly reorganized to make them simpler. This patch is meant to be easy to backport, so C11-style atomics are not used. [BZ #20985] * nptl/Makefile: Adapt. * nptl/pthread_mutex_cond_lock.c (LLL_ROBUST_MUTEX_LOCK): Remove. (LLL_ROBUST_MUTEX_LOCK_MODIFIER): New. * nptl/pthread_mutex_lock.c (LLL_ROBUST_MUTEX_LOCK): Remove. (LLL_ROBUST_MUTEX_LOCK_MODIFIER): New. (__pthread_mutex_lock_full): Inline lll_robust* functions and adapt. * nptl/pthread_mutex_timedlock.c (pthread_mutex_timedlock): Inline lll_robust* functions and adapt. * nptl/pthread_mutex_unlock.c (__pthread_mutex_unlock_full): Likewise. * sysdeps/nptl/lowlevellock.h (__lll_robust_lock_wait, __lll_robust_lock, lll_robust_cond_lock, __lll_robust_timedlock_wait, __lll_robust_timedlock, __lll_robust_unlock): Remove. * sysdeps/unix/sysv/linux/i386/lowlevellock.h (lll_robust_lock, lll_robust_cond_lock, lll_robust_timedlock, lll_robust_unlock): Remove. * sysdeps/unix/sysv/linux/x86_64/lowlevellock.h (lll_robust_lock, lll_robust_cond_lock, lll_robust_timedlock, lll_robust_unlock): Remove. * sysdeps/unix/sysv/linux/sparc/lowlevellock.h (__lll_robust_lock_wait, __lll_robust_lock, lll_robust_cond_lock, __lll_robust_timedlock_wait, __lll_robust_timedlock, __lll_robust_unlock): Remove. * nptl/lowlevelrobustlock.c: Remove file. * nptl/lowlevelrobustlock.sym: Likewise. * sysdeps/unix/sysv/linux/i386/lowlevelrobustlock.S: Likewise. * sysdeps/unix/sysv/linux/x86_64/lowlevelrobustlock.S: Likewise. --- nptl/pthread_mutex_lock.c | 79 +++++++++++++++++++++++++++++++++-------------- 1 file changed, 56 insertions(+), 23 deletions(-) (limited to 'nptl/pthread_mutex_lock.c') diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c index 1795cc13c5..9b81f88a9f 100644 --- a/nptl/pthread_mutex_lock.c +++ b/nptl/pthread_mutex_lock.c @@ -36,14 +36,14 @@ #define lll_trylock_elision(a,t) lll_trylock(a) #endif +/* Some of the following definitions differ when pthread_mutex_cond_lock.c + includes this file. */ #ifndef LLL_MUTEX_LOCK # define LLL_MUTEX_LOCK(mutex) \ lll_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex)) # define LLL_MUTEX_TRYLOCK(mutex) \ lll_trylock ((mutex)->__data.__lock) -# define LLL_ROBUST_MUTEX_LOCK(mutex, id) \ - lll_robust_lock ((mutex)->__data.__lock, id, \ - PTHREAD_ROBUST_MUTEX_PSHARED (mutex)) +# define LLL_ROBUST_MUTEX_LOCK_MODIFIER 0 # define LLL_MUTEX_LOCK_ELISION(mutex) \ lll_lock_elision ((mutex)->__data.__lock, (mutex)->__data.__elision, \ PTHREAD_MUTEX_PSHARED (mutex)) @@ -185,11 +185,21 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex) /* This is set to FUTEX_WAITERS iff we might have shared the FUTEX_WAITERS flag with other threads, and therefore need to keep it set to avoid lost wake-ups. We have the same requirement in the - simple mutex algorithm. */ - unsigned int assume_other_futex_waiters = 0; - do + simple mutex algorithm. + We start with value zero for a normal mutex, and FUTEX_WAITERS if we + are building the special case mutexes for use from within condition + variables. */ + unsigned int assume_other_futex_waiters = LLL_ROBUST_MUTEX_LOCK_MODIFIER; + while (1) { - again: + /* Try to acquire the lock through a CAS from 0 (not acquired) to + our TID | assume_other_futex_waiters. */ + if (__glibc_likely ((oldval == 0) + && (atomic_compare_and_exchange_bool_acq + (&mutex->__data.__lock, + id | assume_other_futex_waiters, 0) == 0))) + break; + if ((oldval & FUTEX_OWNER_DIED) != 0) { /* The previous owner died. Try locking the mutex. */ @@ -209,7 +219,7 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex) if (newval != oldval) { oldval = newval; - goto again; + continue; } /* We got the mutex. */ @@ -260,24 +270,47 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex) } } - oldval = LLL_ROBUST_MUTEX_LOCK (mutex, - id | assume_other_futex_waiters); - /* See above. We set FUTEX_WAITERS and might have shared this flag - with other threads; thus, we need to preserve it. */ - assume_other_futex_waiters = FUTEX_WAITERS; - - if (__builtin_expect (mutex->__data.__owner - == PTHREAD_MUTEX_NOTRECOVERABLE, 0)) + /* We cannot acquire the mutex nor has its owner died. Thus, try + to block using futexes. Set FUTEX_WAITERS if necessary so that + other threads are aware that there are potentially threads + blocked on the futex. Restart if oldval changed in the + meantime. */ + if ((oldval & FUTEX_WAITERS) == 0) { - /* This mutex is now not recoverable. */ - mutex->__data.__count = 0; - lll_unlock (mutex->__data.__lock, - PTHREAD_ROBUST_MUTEX_PSHARED (mutex)); - THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL); - return ENOTRECOVERABLE; + if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock, + oldval | FUTEX_WAITERS, + oldval) + != 0) + { + oldval = mutex->__data.__lock; + continue; + } + oldval |= FUTEX_WAITERS; } + + /* It is now possible that we share the FUTEX_WAITERS flag with + another thread; therefore, update assume_other_futex_waiters so + that we do not forget about this when handling other cases + above and thus do not cause lost wake-ups. */ + assume_other_futex_waiters |= FUTEX_WAITERS; + + /* Block using the futex and reload current lock value. */ + lll_futex_wait (&mutex->__data.__lock, oldval, + PTHREAD_ROBUST_MUTEX_PSHARED (mutex)); + oldval = mutex->__data.__lock; + } + + /* We have acquired the mutex; check if it is still consistent. */ + if (__builtin_expect (mutex->__data.__owner + == PTHREAD_MUTEX_NOTRECOVERABLE, 0)) + { + /* This mutex is now not recoverable. */ + mutex->__data.__count = 0; + int private = PTHREAD_ROBUST_MUTEX_PSHARED (mutex); + lll_unlock (mutex->__data.__lock, private); + THREAD_SETMEM (THREAD_SELF, robust_head.list_op_pending, NULL); + return ENOTRECOVERABLE; } - while ((oldval & FUTEX_OWNER_DIED) != 0); mutex->__data.__count = 1; ENQUEUE_MUTEX (mutex); -- cgit 1.4.1