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authorTorvald Riegel <triegel@redhat.com>2016-12-15 16:06:28 +0100
committerTorvald Riegel <triegel@redhat.com>2016-12-19 20:12:15 +0100
commit353683a22ed8a493a6bd1d78d63e144bc3e85d2f (patch)
tree47077624487b75c819607c8ac2aa2c61654619d0 /nptl
parentda16c9b524908fe0353efc4af8eab6a5ad5ea50b (diff)
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Robust mutexes: Fix lost wake-up.
Assume that Thread 1 waits to acquire a robust mutex using futexes to block (and thus sets the FUTEX_WAITERS flag), and is unblocked when this mutex is released. If Thread 2 concurrently acquires the lock and is killed, Thread 1 can recover from the died owner but fail to restore the FUTEX_WAITERS flag. This can lead to a Thread 3 that also blocked using futexes at the same time as Thread 1 to not get woken up because FUTEX_WAITERS is not set anymore. The fix for this is to ensure that we continue to preserve the FUTEX_WAITERS flag whenever we may have set it or shared it with another thread. This is the same requirement as in the algorithm for normal mutexes, only that the robust mutexes need additional handling for died owners and thus preserving the FUTEX_WAITERS flag cannot be done just in the futex slowpath code. [BZ #20973] * nptl/pthread_mutex_lock.c (__pthread_mutex_lock_full): Fix lost wake-up in robust mutexes. * nptl/pthread_mutex_timedlock.c (pthread_mutex_timedlock): Likewise.
Diffstat (limited to 'nptl')
-rw-r--r--nptl/pthread_mutex_lock.c15
-rw-r--r--nptl/pthread_mutex_timedlock.c14
2 files changed, 25 insertions, 4 deletions
diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index bdfa529..c92bbee 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -182,6 +182,11 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
&mutex->__data.__list.__next);
oldval = mutex->__data.__lock;
+ /* 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
{
again:
@@ -190,9 +195,11 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
/* The previous owner died. Try locking the mutex. */
int newval = id;
#ifdef NO_INCR
+ /* We are not taking assume_other_futex_waiters into accoount
+ here simply because we'll set FUTEX_WAITERS anyway. */
newval |= FUTEX_WAITERS;
#else
- newval |= (oldval & FUTEX_WAITERS);
+ newval |= (oldval & FUTEX_WAITERS) | assume_other_futex_waiters;
#endif
newval
@@ -253,7 +260,11 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
}
}
- oldval = LLL_ROBUST_MUTEX_LOCK (mutex, id);
+ 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))
diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
index 07f0901..21e9eea 100644
--- a/nptl/pthread_mutex_timedlock.c
+++ b/nptl/pthread_mutex_timedlock.c
@@ -142,13 +142,19 @@ pthread_mutex_timedlock (pthread_mutex_t *mutex,
&mutex->__data.__list.__next);
oldval = mutex->__data.__lock;
+ /* 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
{
again:
if ((oldval & FUTEX_OWNER_DIED) != 0)
{
/* The previous owner died. Try locking the mutex. */
- int newval = id | (oldval & FUTEX_WAITERS);
+ int newval = id | (oldval & FUTEX_WAITERS)
+ | assume_other_futex_waiters;
newval
= atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
@@ -203,8 +209,12 @@ pthread_mutex_timedlock (pthread_mutex_t *mutex,
}
}
- result = lll_robust_timedlock (mutex->__data.__lock, abstime, id,
+ result = lll_robust_timedlock (mutex->__data.__lock, abstime,
+ id | assume_other_futex_waiters,
PTHREAD_ROBUST_MUTEX_PSHARED (mutex));
+ /* 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))