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commit 353683a22ed8a493a6bd1d78d63e144bc3e85d2f
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Author: Torvald Riegel <triegel@redhat.com>
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Date: Thu Dec 15 16:06:28 2016 +0100
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Robust mutexes: Fix lost wake-up.
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Assume that Thread 1 waits to acquire a robust mutex using futexes to
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block (and thus sets the FUTEX_WAITERS flag), and is unblocked when this
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mutex is released. If Thread 2 concurrently acquires the lock and is
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killed, Thread 1 can recover from the died owner but fail to restore the
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FUTEX_WAITERS flag. This can lead to a Thread 3 that also blocked using
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futexes at the same time as Thread 1 to not get woken up because
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FUTEX_WAITERS is not set anymore.
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The fix for this is to ensure that we continue to preserve the
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FUTEX_WAITERS flag whenever we may have set it or shared it with another
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thread. This is the same requirement as in the algorithm for normal
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mutexes, only that the robust mutexes need additional handling for died
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owners and thus preserving the FUTEX_WAITERS flag cannot be done just in
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the futex slowpath code.
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[BZ #20973]
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* nptl/pthread_mutex_lock.c (__pthread_mutex_lock_full): Fix lost
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wake-up in robust mutexes.
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* nptl/pthread_mutex_timedlock.c (pthread_mutex_timedlock): Likewise.
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Index: glibc-2.17-c758a686/nptl/pthread_mutex_lock.c
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===================================================================
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--- glibc-2.17-c758a686.orig/nptl/pthread_mutex_lock.c
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+++ glibc-2.17-c758a686/nptl/pthread_mutex_lock.c
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@@ -183,6 +183,11 @@ __pthread_mutex_lock_full (pthread_mutex
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&mutex->__data.__list.__next);
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oldval = mutex->__data.__lock;
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+ /* This is set to FUTEX_WAITERS iff we might have shared the
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+ FUTEX_WAITERS flag with other threads, and therefore need to keep it
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+ set to avoid lost wake-ups. We have the same requirement in the
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+ simple mutex algorithm. */
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+ unsigned int assume_other_futex_waiters = 0;
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do
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{
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again:
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@@ -191,9 +196,11 @@ __pthread_mutex_lock_full (pthread_mutex
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/* The previous owner died. Try locking the mutex. */
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int newval = id;
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#ifdef NO_INCR
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+ /* We are not taking assume_other_futex_waiters into accoount
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+ here simply because we'll set FUTEX_WAITERS anyway. */
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newval |= FUTEX_WAITERS;
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#else
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- newval |= (oldval & FUTEX_WAITERS);
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+ newval |= (oldval & FUTEX_WAITERS) | assume_other_futex_waiters;
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#endif
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newval
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@@ -254,7 +261,11 @@ __pthread_mutex_lock_full (pthread_mutex
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}
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}
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- oldval = LLL_ROBUST_MUTEX_LOCK (mutex, id);
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+ oldval = LLL_ROBUST_MUTEX_LOCK (mutex,
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+ id | assume_other_futex_waiters);
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+ /* See above. We set FUTEX_WAITERS and might have shared this flag
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+ with other threads; thus, we need to preserve it. */
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+ assume_other_futex_waiters = FUTEX_WAITERS;
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if (__builtin_expect (mutex->__data.__owner
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== PTHREAD_MUTEX_NOTRECOVERABLE, 0))
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Index: glibc-2.17-c758a686/nptl/pthread_mutex_timedlock.c
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===================================================================
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--- glibc-2.17-c758a686.orig/nptl/pthread_mutex_timedlock.c
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+++ glibc-2.17-c758a686/nptl/pthread_mutex_timedlock.c
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@@ -142,13 +142,19 @@ pthread_mutex_timedlock (pthread_mutex_t
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&mutex->__data.__list.__next);
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oldval = mutex->__data.__lock;
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+ /* This is set to FUTEX_WAITERS iff we might have shared the
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+ FUTEX_WAITERS flag with other threads, and therefore need to keep it
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+ set to avoid lost wake-ups. We have the same requirement in the
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+ simple mutex algorithm. */
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+ unsigned int assume_other_futex_waiters = 0;
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do
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{
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again:
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if ((oldval & FUTEX_OWNER_DIED) != 0)
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{
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/* The previous owner died. Try locking the mutex. */
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- int newval = id | (oldval & FUTEX_WAITERS);
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+ int newval = id | (oldval & FUTEX_WAITERS)
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+ | assume_other_futex_waiters;
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newval
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= atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
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@@ -203,8 +209,12 @@ pthread_mutex_timedlock (pthread_mutex_t
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}
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}
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- result = lll_robust_timedlock (mutex->__data.__lock, abstime, id,
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+ result = lll_robust_timedlock (mutex->__data.__lock, abstime,
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+ id | assume_other_futex_waiters,
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PTHREAD_ROBUST_MUTEX_PSHARED (mutex));
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+ /* See above. We set FUTEX_WAITERS and might have shared this flag
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+ with other threads; thus, we need to preserve it. */
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+ assume_other_futex_waiters = FUTEX_WAITERS;
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if (__builtin_expect (mutex->__data.__owner
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== PTHREAD_MUTEX_NOTRECOVERABLE, 0))
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