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bpo-41710: PyThread_acquire_lock_timed() uses sem_clockwait() #28662

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Merged
merged 3 commits into from
Oct 1, 2021
Merged

bpo-41710: PyThread_acquire_lock_timed() uses sem_clockwait() #28662

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058de0e
bpo-41710: PyThread_acquire_lock_timed() uses sem_clockwait()
vstinner Sep 30, 2021
ced6f76
Compute the absolute timeout once outside the loop
vstinner Sep 30, 2021
6c4774e
Fix typo
vstinner Oct 1, 2021
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10 changes: 10 additions & 0 deletions Doc/whatsnew/3.11.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -239,6 +239,16 @@ sqlite3
(Contributed by Aviv Palivoda, Daniel Shahaf, and Erlend E. Aasland in
:issue:`16379`.)

threading
---------

* On Unix, if the ``sem_clockwait()`` function is available in the C library
(glibc 2.30 and newer), the :meth:`threading.Lock.acquire` method now uses
the monotonic clock (:data:`time.CLOCK_MONOTONIC`) for the timeout, rather
than using the system clock (:data:`time.CLOCK_REALTIME`), to not be affected
by system clock changes.
(Contributed by Livius and Victor Stinner in :issue:`41710`.)

time
----

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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Library/2021-09-30-23-00-18.bpo-41710.svuloZ.rst
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@@ -0,0 +1,5 @@
On Unix, if the ``sem_clockwait()`` function is available in the C library
(glibc 2.30 and newer), the :meth:`threading.Lock.acquire` method now uses the
monotonic clock (:data:`time.CLOCK_MONOTONIC`) for the timeout, rather than
using the system clock (:data:`time.CLOCK_REALTIME`), to not be affected by
system clock changes. Patch by Victor Stinner.
39 changes: 33 additions & 6 deletions Python/thread_pthread.h
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Original file line number Diff line number Diff line change
Expand Up @@ -92,7 +92,7 @@
* mutexes and condition variables:
*/
#if (defined(_POSIX_SEMAPHORES) && !defined(HAVE_BROKEN_POSIX_SEMAPHORES) && \
defined(HAVE_SEM_TIMEDWAIT))
(defined(HAVE_SEM_TIMEDWAIT) || defined(HAVE_SEM_CLOCKWAIT)))
# define USE_SEMAPHORES
#else
# undef USE_SEMAPHORES
Expand Down Expand Up @@ -461,17 +461,34 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
timeout = _PyTime_FromNanoseconds(-1);
}

#ifdef HAVE_SEM_CLOCKWAIT
struct timespec abs_timeout;
// Local scope for deadline
{
_PyTime_t deadline = _PyTime_GetMonotonicClock() + timeout;
_PyTime_AsTimespec_clamp(deadline, &abs_timeout);
}
#else
_PyTime_t deadline = 0;
if (timeout > 0 && !intr_flag) {
if (timeout > 0
&& !intr_flag
)
{
deadline = _PyTime_GetMonotonicClock() + timeout;
}
#endif

while (1) {
if (timeout > 0) {
_PyTime_t t = _PyTime_GetSystemClock() + timeout;
#ifdef HAVE_SEM_CLOCKWAIT
status = fix_status(sem_clockwait(thelock, CLOCK_MONOTONIC,
&abs_timeout));
#else
_PyTime_t abs_timeout = _PyTime_GetSystemClock() + timeout;
struct timespec ts;
_PyTime_AsTimespec_clamp(t, &ts);
_PyTime_AsTimespec_clamp(abs_timeout, &ts);
status = fix_status(sem_timedwait(thelock, &ts));
#endif
}
else if (timeout == 0) {
status = fix_status(sem_trywait(thelock));
Expand All @@ -486,6 +503,9 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
break;
}

// sem_clockwait() uses an absolution timeout, there is no need
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s/absolution/absolute

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Nobody expects the Spanish Absolution!

// to recompute the relative timeout.
#ifndef HAVE_SEM_CLOCKWAIT
if (timeout > 0) {
/* wait interrupted by a signal (EINTR): recompute the timeout */
_PyTime_t timeout = deadline - _PyTime_GetMonotonicClock();
Expand All @@ -494,17 +514,24 @@ PyThread_acquire_lock_timed(PyThread_type_lock lock, PY_TIMEOUT_T microseconds,
break;
}
}
#endif
}

/* Don't check the status if we're stopping because of an interrupt. */
if (!(intr_flag && status == EINTR)) {
if (timeout > 0) {
if (status != ETIMEDOUT)
if (status != ETIMEDOUT) {
#ifdef HAVE_SEM_CLOCKWAIT
CHECK_STATUS("sem_clockwait");
#else
CHECK_STATUS("sem_timedwait");
#endif
}
}
else if (timeout == 0) {
if (status != EAGAIN)
if (status != EAGAIN) {
CHECK_STATUS("sem_trywait");
}
}
else {
CHECK_STATUS("sem_wait");
Expand Down
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