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[2.7] bpo-38106: Fix race in PyThread_release_lock that can lead to memory corruption and deadlock #16047

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Merged
vstinner merged 1 commit into from
Oct 3, 2019
Merged

[2.7] bpo-38106: Fix race in PyThread_release_lock that can lead to memory corruption and deadlock #16047

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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2019-09-12-16-36-16.bpo-38106.4pApn7.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
Fix race in PyThread_release_lock that was leading to memory corruption and
deadlocks. The fix applies to POSIX systems where Python locks are implemented
with mutex and condition variable because POSIX semaphores are either not
provided, or are known to be broken. One particular example of such system is
macOS.
15 changes: 9 additions & 6 deletions Python/thread_pthread.h
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Original file line number Diff line number Diff line change
Expand Up @@ -430,12 +430,15 @@ PyThread_free_lock(PyThread_type_lock lock)
(void) error; /* silence unused-but-set-variable warning */
dprintf(("PyThread_free_lock(%p) called\n", lock));

status = pthread_mutex_destroy( &thelock->mut );
CHECK_STATUS("pthread_mutex_destroy");

/* some pthread-like implementations tie the mutex to the cond
* and must have the cond destroyed first.
*/
status = pthread_cond_destroy( &thelock->lock_released );
CHECK_STATUS("pthread_cond_destroy");

status = pthread_mutex_destroy( &thelock->mut );
CHECK_STATUS("pthread_mutex_destroy");

free((void *)thelock);
}

Expand Down Expand Up @@ -497,12 +500,12 @@ PyThread_release_lock(PyThread_type_lock lock)

thelock->locked = 0;

status = pthread_mutex_unlock( &thelock->mut );
CHECK_STATUS("pthread_mutex_unlock[3]");

/* wake up someone (anyone, if any) waiting on the lock */
status = pthread_cond_signal( &thelock->lock_released );
CHECK_STATUS("pthread_cond_signal");

status = pthread_mutex_unlock( &thelock->mut );
CHECK_STATUS("pthread_mutex_unlock[3]");
}

#endif /* USE_SEMAPHORES */
Expand Down