356 lines
12 KiB
C
356 lines
12 KiB
C
|
/* Bug 23844: Test for pthread_rwlock_tryrdlock stalls.
|
||
|
|
Copyright (C) 2019-2022 Free Software Foundation, Inc.
|
||
|
|
This file is part of the GNU C Library.
|
||
|
|
|
||
|
|
The GNU C Library is free software; you can redistribute it and/or
|
||
|
|
modify it under the terms of the GNU Lesser General Public
|
||
|
|
License as published by the Free Software Foundation; either
|
||
|
|
version 2.1 of the License, or (at your option) any later version.
|
||
|
|
|
||
|
|
The GNU C Library is distributed in the hope that it will be useful,
|
||
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
||
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
||
|
|
Lesser General Public License for more details.
|
||
|
|
|
||
|
|
You should have received a copy of the GNU Lesser General Public
|
||
|
|
License along with the GNU C Library; if not, see
|
||
|
|
<https://www.gnu.org/licenses/>. */
|
||
|
|
|
||
|
|
/* For a full analysis see comment:
|
||
|
|
https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14
|
||
|
|
|
||
|
|
Provided here for reference:
|
||
|
|
|
||
|
|
--- Analysis of pthread_rwlock_tryrdlock() stall ---
|
||
|
|
A read lock begins to execute.
|
||
|
|
|
||
|
|
In __pthread_rwlock_rdlock_full:
|
||
|
|
|
||
|
|
We can attempt a read lock, but find that the lock is
|
||
|
|
in a write phase (PTHREAD_RWLOCK_WRPHASE, or WP-bit
|
||
|
|
is set), and the lock is held by a primary writer
|
||
|
|
(PTHREAD_RWLOCK_WRLOCKED is set). In this case we must
|
||
|
|
wait for explicit hand over from the writer to us or
|
||
|
|
one of the other waiters. The read lock threads are
|
||
|
|
about to execute:
|
||
|
|
|
||
|
|
341 r = (atomic_fetch_add_acquire (&rwlock->__data.__readers,
|
||
|
|
342 (1 << PTHREAD_RWLOCK_READER_SHIFT))
|
||
|
|
343 + (1 << PTHREAD_RWLOCK_READER_SHIFT));
|
||
|
|
|
||
|
|
An unlock beings to execute.
|
||
|
|
|
||
|
|
Then in __pthread_rwlock_wrunlock:
|
||
|
|
|
||
|
|
547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
|
||
|
|
...
|
||
|
|
549 while (!atomic_compare_exchange_weak_release
|
||
|
|
550 (&rwlock->__data.__readers, &r,
|
||
|
|
551 ((r ^ PTHREAD_RWLOCK_WRLOCKED)
|
||
|
|
552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
|
||
|
|
553 : PTHREAD_RWLOCK_WRPHASE))))
|
||
|
|
554 {
|
||
|
|
...
|
||
|
|
556 }
|
||
|
|
|
||
|
|
We clear PTHREAD_RWLOCK_WRLOCKED, and if there are
|
||
|
|
no readers so we leave the lock in PTHRAD_RWLOCK_WRPHASE.
|
||
|
|
|
||
|
|
Back in the read lock.
|
||
|
|
|
||
|
|
The read lock adjusts __readres as above.
|
||
|
|
|
||
|
|
383 while ((r & PTHREAD_RWLOCK_WRPHASE) != 0
|
||
|
|
384 && (r & PTHREAD_RWLOCK_WRLOCKED) == 0)
|
||
|
|
385 {
|
||
|
|
...
|
||
|
|
390 if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
|
||
|
|
391 r ^ PTHREAD_RWLOCK_WRPHASE))
|
||
|
|
392 {
|
||
|
|
|
||
|
|
And then attemps to start the read phase.
|
||
|
|
|
||
|
|
Assume there happens to be a tryrdlock at this point, noting
|
||
|
|
that PTHREAD_RWLOCK_WRLOCKED is clear, and PTHREAD_RWLOCK_WRPHASE
|
||
|
|
is 1. So the try lock attemps to start the read phase.
|
||
|
|
|
||
|
|
In __pthread_rwlock_tryrdlock:
|
||
|
|
|
||
|
|
44 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
|
||
|
|
45 {
|
||
|
|
...
|
||
|
|
49 if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
|
||
|
|
50 && (rwlock->__data.__flags
|
||
|
|
51 == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
|
||
|
|
52 return EBUSY;
|
||
|
|
53 rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
|
||
|
|
54 }
|
||
|
|
...
|
||
|
|
89 while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
|
||
|
|
90 &r, rnew));
|
||
|
|
|
||
|
|
And succeeds.
|
||
|
|
|
||
|
|
Back in the write unlock:
|
||
|
|
|
||
|
|
557 if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
|
||
|
|
558 {
|
||
|
|
...
|
||
|
|
563 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
|
||
|
|
564 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
|
||
|
|
565 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
|
||
|
|
566 }
|
||
|
|
|
||
|
|
We note that PTHREAD_RWLOCK_FUTEX_USED is non-zero
|
||
|
|
and don't wake anyone. This is OK because we handed
|
||
|
|
over to the trylock. It will be the trylock's responsibility
|
||
|
|
to wake any waiters.
|
||
|
|
|
||
|
|
Back in the read lock:
|
||
|
|
|
||
|
|
The read lock fails to install PTHRAD_REWLOCK_WRPHASE as 0 because
|
||
|
|
the __readers value was adjusted by the trylock, and so it falls through
|
||
|
|
to waiting on the lock for explicit handover from either a new writer
|
||
|
|
or a new reader.
|
||
|
|
|
||
|
|
448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
|
||
|
|
449 1 | PTHREAD_RWLOCK_FUTEX_USED,
|
||
|
|
450 abstime, private);
|
||
|
|
|
||
|
|
We use PTHREAD_RWLOCK_FUTEX_USED to indicate the futex
|
||
|
|
is in use.
|
||
|
|
|
||
|
|
At this point we have readers waiting on the read lock
|
||
|
|
to unlock. The wrlock is done. The trylock is finishing
|
||
|
|
the installation of the read phase.
|
||
|
|
|
||
|
|
92 if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
|
||
|
|
93 {
|
||
|
|
...
|
||
|
|
105 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
|
||
|
|
106 }
|
||
|
|
|
||
|
|
The trylock does note that we were the one that
|
||
|
|
installed the read phase, but the comments are not
|
||
|
|
correct, the execution ordering above shows that
|
||
|
|
readers might indeed be waiting, and they are.
|
||
|
|
|
||
|
|
The atomic_store_relaxed throws away PTHREAD_RWLOCK_FUTEX_USED,
|
||
|
|
and the waiting reader is never worken becuase as noted
|
||
|
|
above it is conditional on the futex being used.
|
||
|
|
|
||
|
|
The solution is for the trylock thread to inspect
|
||
|
|
PTHREAD_RWLOCK_FUTEX_USED and wake the waiting readers.
|
||
|
|
|
||
|
|
--- Analysis of pthread_rwlock_trywrlock() stall ---
|
||
|
|
|
||
|
|
A write lock begins to execute, takes the write lock,
|
||
|
|
and then releases the lock...
|
||
|
|
|
||
|
|
In pthread_rwlock_wrunlock():
|
||
|
|
|
||
|
|
547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
|
||
|
|
...
|
||
|
|
549 while (!atomic_compare_exchange_weak_release
|
||
|
|
550 (&rwlock->__data.__readers, &r,
|
||
|
|
551 ((r ^ PTHREAD_RWLOCK_WRLOCKED)
|
||
|
|
552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
|
||
|
|
553 : PTHREAD_RWLOCK_WRPHASE))))
|
||
|
|
554 {
|
||
|
|
...
|
||
|
|
556 }
|
||
|
|
|
||
|
|
... leaving it in the write phase with zero readers
|
||
|
|
(the case where we leave the write phase in place
|
||
|
|
during a write unlock).
|
||
|
|
|
||
|
|
A write trylock begins to execute.
|
||
|
|
|
||
|
|
In __pthread_rwlock_trywrlock:
|
||
|
|
|
||
|
|
40 while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
|
||
|
|
41 && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
|
||
|
|
42 || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
|
||
|
|
43 {
|
||
|
|
|
||
|
|
The lock is not locked.
|
||
|
|
|
||
|
|
There are no readers.
|
||
|
|
|
||
|
|
45 if (atomic_compare_exchange_weak_acquire (
|
||
|
|
46 &rwlock->__data.__readers, &r,
|
||
|
|
47 r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
|
||
|
|
|
||
|
|
We atomically install the write phase and we take the
|
||
|
|
exclusive write lock.
|
||
|
|
|
||
|
|
48 {
|
||
|
|
49 atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
|
||
|
|
|
||
|
|
We get this far.
|
||
|
|
|
||
|
|
A reader lock begins to execute.
|
||
|
|
|
||
|
|
In pthread_rwlock_rdlock:
|
||
|
|
|
||
|
|
437 for (;;)
|
||
|
|
438 {
|
||
|
|
439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
|
||
|
|
440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
|
||
|
|
441 {
|
||
|
|
442 int private = __pthread_rwlock_get_private (rwlock);
|
||
|
|
443 if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
|
||
|
|
444 && (!atomic_compare_exchange_weak_relaxed
|
||
|
|
445 (&rwlock->__data.__wrphase_futex,
|
||
|
|
446 &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED)))
|
||
|
|
447 continue;
|
||
|
|
448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
|
||
|
|
449 1 | PTHREAD_RWLOCK_FUTEX_USED,
|
||
|
|
450 abstime, private);
|
||
|
|
|
||
|
|
We are in a write phase, so the while() on line 439 is true.
|
||
|
|
|
||
|
|
The value of wpf does not have PTHREAD_RWLOCK_FUTEX_USED set
|
||
|
|
since this is the first reader to lock.
|
||
|
|
|
||
|
|
The atomic operation sets wpf with PTHREAD_RELOCK_FUTEX_USED
|
||
|
|
on the expectation that this reader will be woken during
|
||
|
|
the handoff.
|
||
|
|
|
||
|
|
Back in pthread_rwlock_trywrlock:
|
||
|
|
|
||
|
|
50 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
|
||
|
|
51 atomic_store_relaxed (&rwlock->__data.__cur_writer,
|
||
|
|
52 THREAD_GETMEM (THREAD_SELF, tid));
|
||
|
|
53 return 0;
|
||
|
|
54 }
|
||
|
|
...
|
||
|
|
57 }
|
||
|
|
|
||
|
|
We write 1 to __wrphase_futex discarding PTHREAD_RWLOCK_FUTEX_USED,
|
||
|
|
and so in the unlock we will not awaken the waiting reader.
|
||
|
|
|
||
|
|
The solution to this is to realize that if we did not start the write
|
||
|
|
phase we need not write 1 or any other value to __wrphase_futex.
|
||
|
|
This ensures that any readers (which saw __wrphase_futex != 0) can
|
||
|
|
set PTHREAD_RWLOCK_FUTEX_USED and this can be used at unlock to
|
||
|
|
wake them.
|
||
|
|
|
||
|
|
If we installed the write phase then all other readers are looping
|
||
|
|
here:
|
||
|
|
|
||
|
|
In __pthread_rwlock_rdlock_full:
|
||
|
|
|
||
|
|
437 for (;;)
|
||
|
|
438 {
|
||
|
|
439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
|
||
|
|
440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
|
||
|
|
441 {
|
||
|
|
...
|
||
|
|
508 }
|
||
|
|
|
||
|
|
waiting for the write phase to be installed or removed before they
|
||
|
|
can begin waiting on __wrphase_futex (part of the algorithm), or
|
||
|
|
taking a concurrent read lock, and thus we can safely write 1 to
|
||
|
|
__wrphase_futex.
|
||
|
|
|
||
|
|
If we did not install the write phase then the readers may already
|
||
|
|
be waiting on the futex, the original writer wrote 1 to __wrphase_futex
|
||
|
|
as part of starting the write phase, and we cannot also write 1
|
||
|
|
without loosing the PTHREAD_RWLOCK_FUTEX_USED bit.
|
||
|
|
|
||
|
|
---
|
||
|
|
|
||
|
|
Summary for the pthread_rwlock_tryrdlock() stall:
|
||
|
|
|
||
|
|
The stall is caused by pthread_rwlock_tryrdlock failing to check
|
||
|
|
that PTHREAD_RWLOCK_FUTEX_USED is set in the __wrphase_futex futex
|
||
|
|
and then waking the futex.
|
||
|
|
|
||
|
|
The fix for bug 23844 ensures that waiters on __wrphase_futex are
|
||
|
|
correctly woken. Before the fix the test stalls as readers can
|
||
|
|
wait forever on __wrphase_futex. */
|
||
|
|
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <stdlib.h>
|
||
|
|
#include <unistd.h>
|
||
|
|
#include <pthread.h>
|
||
|
|
#include <support/xthread.h>
|
||
|
|
#include <errno.h>
|
||
|
|
|
||
|
|
/* We need only one lock to reproduce the issue. We will need multiple
|
||
|
|
threads to get the exact case where we have a read, try, and unlock
|
||
|
|
all interleaving to produce the case where the readers are waiting
|
||
|
|
and the try fails to wake them. */
|
||
|
|
pthread_rwlock_t onelock;
|
||
|
|
|
||
|
|
/* The number of threads is arbitrary but empirically chosen to have
|
||
|
|
enough threads that we see the condition where waiting readers are
|
||
|
|
not woken by a successful tryrdlock. */
|
||
|
|
#define NTHREADS 32
|
||
|
|
|
||
|
|
_Atomic int do_exit;
|
||
|
|
|
||
|
|
void *
|
||
|
|
run_loop (void *arg)
|
||
|
|
{
|
||
|
|
int i = 0, ret;
|
||
|
|
while (!do_exit)
|
||
|
|
{
|
||
|
|
/* Arbitrarily choose if we are the writer or reader. Choose a
|
||
|
|
high enough ratio of readers to writers to make it likely
|
||
|
|
that readers block (and eventually are susceptable to
|
||
|
|
stalling).
|
||
|
|
|
||
|
|
If we are a writer, take the write lock, and then unlock.
|
||
|
|
If we are a reader, try the lock, then lock, then unlock. */
|
||
|
|
if ((i % 8) != 0)
|
||
|
|
xpthread_rwlock_wrlock (&onelock);
|
||
|
|
else
|
||
|
|
{
|
||
|
|
if ((ret = pthread_rwlock_tryrdlock (&onelock)) != 0)
|
||
|
|
{
|
||
|
|
if (ret == EBUSY)
|
||
|
|
xpthread_rwlock_rdlock (&onelock);
|
||
|
|
else
|
||
|
|
exit (EXIT_FAILURE);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
/* Thread does some work and then unlocks. */
|
||
|
|
xpthread_rwlock_unlock (&onelock);
|
||
|
|
i++;
|
||
|
|
}
|
||
|
|
return NULL;
|
||
|
|
}
|
||
|
|
|
||
|
|
int
|
||
|
|
do_test (void)
|
||
|
|
{
|
||
|
|
int i;
|
||
|
|
pthread_t tids[NTHREADS];
|
||
|
|
xpthread_rwlock_init (&onelock, NULL);
|
||
|
|
for (i = 0; i < NTHREADS; i++)
|
||
|
|
tids[i] = xpthread_create (NULL, run_loop, NULL);
|
||
|
|
/* Run for some amount of time. Empirically speaking exercising
|
||
|
|
the stall via pthread_rwlock_tryrdlock is much harder, and on
|
||
|
|
a 3.5GHz 4 core x86_64 VM system it takes somewhere around
|
||
|
|
20-200s to stall, approaching 100% stall past 200s. We can't
|
||
|
|
wait that long for a regression test so we just test for 20s,
|
||
|
|
and expect the stall to happen with a 5-10% chance (enough for
|
||
|
|
developers to see). */
|
||
|
|
sleep (20);
|
||
|
|
/* Then exit. */
|
||
|
|
printf ("INFO: Exiting...\n");
|
||
|
|
do_exit = 1;
|
||
|
|
/* If any readers stalled then we will timeout waiting for them. */
|
||
|
|
for (i = 0; i < NTHREADS; i++)
|
||
|
|
xpthread_join (tids[i]);
|
||
|
|
printf ("INFO: Done.\n");
|
||
|
|
xpthread_rwlock_destroy (&onelock);
|
||
|
|
printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n");
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
#define TIMEOUT 30
|
||
|
|
#include <support/test-driver.c>
|