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bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to #2403

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Merged
pitrou merged 4 commits into from
Jun 26, 2017
Merged

bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not to #2403

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bpo-30765: Avoid blocking when PyThread_acquire_lock() is asked not t…
pitrou Jun 26, 2017
03ba2b4
Update 2017-06-26-14-29-50.bpo-30765.Q5iBmf.rst
pitrou Jun 26, 2017
5d4b232
Avoid core logic when taking the mutex failed
pitrou Jun 26, 2017
59973f1
Merge branch 'mutex_trylock' of github.com:pitrou/cpython into mutex_…
pitrou Jun 26, 2017
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2017-06-26-14-29-50.bpo-30765.Q5iBmf.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Avoid blocking in pthread_mutex_lock() when PyThread_acquire_lock() is asked
not to block.
13 changes: 10 additions & 3 deletions Python/thread_pthread.h
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Original file line number Diff line number Diff line change
Expand Up @@ -473,10 +473,17 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
dprintf(("PyThread_acquire_lock_timed(%p, %lld, %d) called\n",
lock, microseconds, intr_flag));

status = pthread_mutex_lock( &thelock->mut );
CHECK_STATUS_PTHREAD("pthread_mutex_lock[1]");
if (microseconds == 0) {
status = pthread_mutex_trylock( &thelock->mut );
if (status != EBUSY)
CHECK_STATUS_PTHREAD("pthread_mutex_trylock[1]");

@vstinner vstinner Jun 26, 2017

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I don't understand how these locks work. Can you please explain in a comment what happens when we fail to lock the mutex? Is it safe to call pthread_mutex_unlock() anyway?

@pitrou pitrou Jun 26, 2017

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You're right that we may want to fail early when taking the mutex fails.

}
else {
status = pthread_mutex_lock( &thelock->mut );
CHECK_STATUS_PTHREAD("pthread_mutex_lock[1]");
}

if (thelock->locked == 0) {
if (status == 0 && thelock->locked == 0) {
success = PY_LOCK_ACQUIRED;
} else if (microseconds == 0) {
success = PY_LOCK_FAILURE;
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