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PyThread_acquire_lock_timed() should recompute the timeout when interrupted by a signal #74953
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The current code of PyThread_acquire_lock_timed() (the implementation not using semaphore) doesn't compute correctly the timeout when pthread_cond_timedwait() is interrupted by a signal. We should recompute the timeout using a deadline.
Something like select.select():
if (tvp)
deadline = _PyTime_GetMonotonicClock() + timeout;do {
... use tvp
if (errno != EINTR)
break;/* select() was interrupted by a signal */ if (PyErr_CheckSignals()) goto finally;if (tvp) { timeout = deadline - _PyTime_GetMonotonicClock(); if (timeout < 0) { n = 0; break; } _PyTime_AsTimeval_noraise(timeout, &tv, _PyTime_ROUND_CEILING); /* retry select() with the recomputed timeout */ } } while (1);
interrupted_lock.py: test threading.Lock.acquire(timeout=1.0) with SIGALRM sent every 1 ms (so up to 1000 times in total). Example:
haypo@selma$ ./python interrupted_lock.py
acquire(timeout=1.0) took 1.0 seconds and got 1000 signalsOh, in fact, threading.Lock.acquire(timeout=1.0) already recomputes the timeout when interrupted.
In Python stdlib, PyThread_acquire_lock_timed() is only called from one place with intr_flag=0: faulthandler watchdog thread, but this thread blocks all signals:
/* we don't want to receive any signal */ sigfillset(&set); pthread_sigmask(SIG_SETMASK, &set, NULL);Ah, I found another caller of PyThread_acquire_lock_timed() with a timeout > 0 and intr_flag=0: _enter_buffered_busy() of Modules/_io/bufferedio.c:
/* When finalizing, we don't want a deadlock to happen with daemon * threads abruptly shut down while they owned the lock. * Therefore, only wait for a grace period (1 s.). * Note that non-daemon threads have already exited here, so this * shouldn't affect carefully written threaded I/O code. */ st = PyThread_acquire_lock_timed(self->lock, (PY_TIMEOUT_T)1e6, 0);I wrote PR 4103 to fix the pthread+semaphore implementation of PyThread_acquire_lock_timed().
Apply PR 4103, apply attached test.patch, recompile Python, and run interrupted_lock.py to test the PR. Result:
haypo@selma$ ./python interrupted_lock.py
acquire(timeout=1.0) took 1.0 seconds and got 911 signalsTo check if you are using pthread+semaphore, use:
haypo@selma$ ./python -c 'import sys; print(sys.thread_info)'
sys.thread_info(name='pthread', lock='semaphore', version='NPTL 2.25')Here you have pthread+semaphore. It's Fedora 26 running Linux kernel 4.13.
I merged my PR. Thanks Antoine Pitrou for the review!
This change only impacts the io.BufferedWriter and io.BufferedReader during Python finalization. It has no effect on theading.Lock.acquire(). It might impact faulthandler.dump_traceback_later(), but in practice, it shouldn't change anything since the internal faulthandler watchdog thread blocks all signals.
If I understand correctly, if the system clock is stepped back by 1 hour during Python finalization, after PyThread_acquire_lock_timed() computed the deadline, but before sem_timedwait() completed, _enter_buffered_busy() can be blocked during 1 hour.
Moving the system clock backward by 1 hour occurs once a year on the DST change. But the race condition is unlikely since the size of the time window is only 1 second. The DST change should occur at Python shutdown when an io.BufferedReader or io.BufferedWriter is used.
Since the race condition seems very unlikely and was never reported by another user, I propose to not backward this change. Moreover, I'm not confident to modify locks in a stable release :-)
Oh, the pthread condvar+mutex implementation still has the bug, so I reopen the issue.
- added a commit that references this issue
on Jun 19, 2022 I looked again to this issue. IMO it's now fully fixed, see my comment for the rationale: #93946 (comment)
- added a commit that references this issue
on Jun 21, 2022 - added a commit that references this issue
on Jun 26, 2022
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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