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356 lines
12 KiB
C
356 lines
12 KiB
C
/* Bug 23844: Test for pthread_rwlock_tryrdlock stalls.
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Copyright (C) 2019-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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/* For a full analysis see comment:
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https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14
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Provided here for reference:
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--- Analysis of pthread_rwlock_tryrdlock() stall ---
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A read lock begins to execute.
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In __pthread_rwlock_rdlock_full:
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We can attempt a read lock, but find that the lock is
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in a write phase (PTHREAD_RWLOCK_WRPHASE, or WP-bit
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is set), and the lock is held by a primary writer
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(PTHREAD_RWLOCK_WRLOCKED is set). In this case we must
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wait for explicit hand over from the writer to us or
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one of the other waiters. The read lock threads are
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about to execute:
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341 r = (atomic_fetch_add_acquire (&rwlock->__data.__readers,
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342 (1 << PTHREAD_RWLOCK_READER_SHIFT))
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343 + (1 << PTHREAD_RWLOCK_READER_SHIFT));
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An unlock beings to execute.
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Then in __pthread_rwlock_wrunlock:
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547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
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...
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549 while (!atomic_compare_exchange_weak_release
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550 (&rwlock->__data.__readers, &r,
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551 ((r ^ PTHREAD_RWLOCK_WRLOCKED)
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552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
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553 : PTHREAD_RWLOCK_WRPHASE))))
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554 {
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...
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556 }
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We clear PTHREAD_RWLOCK_WRLOCKED, and if there are
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no readers so we leave the lock in PTHRAD_RWLOCK_WRPHASE.
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Back in the read lock.
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The read lock adjusts __readres as above.
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383 while ((r & PTHREAD_RWLOCK_WRPHASE) != 0
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384 && (r & PTHREAD_RWLOCK_WRLOCKED) == 0)
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385 {
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...
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390 if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
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391 r ^ PTHREAD_RWLOCK_WRPHASE))
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392 {
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And then attemps to start the read phase.
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Assume there happens to be a tryrdlock at this point, noting
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that PTHREAD_RWLOCK_WRLOCKED is clear, and PTHREAD_RWLOCK_WRPHASE
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is 1. So the try lock attemps to start the read phase.
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In __pthread_rwlock_tryrdlock:
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44 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
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45 {
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...
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49 if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
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50 && (rwlock->__data.__flags
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51 == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
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52 return EBUSY;
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53 rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
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54 }
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...
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89 while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
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90 &r, rnew));
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And succeeds.
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Back in the write unlock:
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557 if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
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558 {
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...
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563 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
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564 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
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565 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
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566 }
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We note that PTHREAD_RWLOCK_FUTEX_USED is non-zero
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and don't wake anyone. This is OK because we handed
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over to the trylock. It will be the trylock's responsibility
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to wake any waiters.
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Back in the read lock:
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The read lock fails to install PTHRAD_REWLOCK_WRPHASE as 0 because
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the __readers value was adjusted by the trylock, and so it falls through
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to waiting on the lock for explicit handover from either a new writer
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or a new reader.
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448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
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449 1 | PTHREAD_RWLOCK_FUTEX_USED,
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450 abstime, private);
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We use PTHREAD_RWLOCK_FUTEX_USED to indicate the futex
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is in use.
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At this point we have readers waiting on the read lock
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to unlock. The wrlock is done. The trylock is finishing
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the installation of the read phase.
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92 if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
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93 {
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...
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105 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
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106 }
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The trylock does note that we were the one that
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installed the read phase, but the comments are not
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correct, the execution ordering above shows that
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readers might indeed be waiting, and they are.
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The atomic_store_relaxed throws away PTHREAD_RWLOCK_FUTEX_USED,
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and the waiting reader is never worken becuase as noted
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above it is conditional on the futex being used.
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The solution is for the trylock thread to inspect
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PTHREAD_RWLOCK_FUTEX_USED and wake the waiting readers.
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--- Analysis of pthread_rwlock_trywrlock() stall ---
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A write lock begins to execute, takes the write lock,
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and then releases the lock...
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In pthread_rwlock_wrunlock():
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547 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
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...
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549 while (!atomic_compare_exchange_weak_release
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550 (&rwlock->__data.__readers, &r,
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551 ((r ^ PTHREAD_RWLOCK_WRLOCKED)
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552 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
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553 : PTHREAD_RWLOCK_WRPHASE))))
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554 {
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...
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556 }
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... leaving it in the write phase with zero readers
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(the case where we leave the write phase in place
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during a write unlock).
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A write trylock begins to execute.
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In __pthread_rwlock_trywrlock:
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40 while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
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41 && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
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42 || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
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43 {
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The lock is not locked.
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There are no readers.
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45 if (atomic_compare_exchange_weak_acquire (
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46 &rwlock->__data.__readers, &r,
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47 r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
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We atomically install the write phase and we take the
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exclusive write lock.
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48 {
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49 atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
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We get this far.
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A reader lock begins to execute.
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In pthread_rwlock_rdlock:
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437 for (;;)
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438 {
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439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
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440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
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441 {
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442 int private = __pthread_rwlock_get_private (rwlock);
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443 if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
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444 && (!atomic_compare_exchange_weak_relaxed
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445 (&rwlock->__data.__wrphase_futex,
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446 &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED)))
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447 continue;
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448 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
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449 1 | PTHREAD_RWLOCK_FUTEX_USED,
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450 abstime, private);
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We are in a write phase, so the while() on line 439 is true.
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The value of wpf does not have PTHREAD_RWLOCK_FUTEX_USED set
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since this is the first reader to lock.
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The atomic operation sets wpf with PTHREAD_RELOCK_FUTEX_USED
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on the expectation that this reader will be woken during
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the handoff.
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Back in pthread_rwlock_trywrlock:
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50 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
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51 atomic_store_relaxed (&rwlock->__data.__cur_writer,
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52 THREAD_GETMEM (THREAD_SELF, tid));
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53 return 0;
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54 }
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...
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57 }
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We write 1 to __wrphase_futex discarding PTHREAD_RWLOCK_FUTEX_USED,
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and so in the unlock we will not awaken the waiting reader.
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The solution to this is to realize that if we did not start the write
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phase we need not write 1 or any other value to __wrphase_futex.
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This ensures that any readers (which saw __wrphase_futex != 0) can
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set PTHREAD_RWLOCK_FUTEX_USED and this can be used at unlock to
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wake them.
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If we installed the write phase then all other readers are looping
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here:
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In __pthread_rwlock_rdlock_full:
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437 for (;;)
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438 {
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439 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
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440 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
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441 {
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...
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508 }
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waiting for the write phase to be installed or removed before they
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can begin waiting on __wrphase_futex (part of the algorithm), or
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taking a concurrent read lock, and thus we can safely write 1 to
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__wrphase_futex.
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If we did not install the write phase then the readers may already
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be waiting on the futex, the original writer wrote 1 to __wrphase_futex
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as part of starting the write phase, and we cannot also write 1
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without loosing the PTHREAD_RWLOCK_FUTEX_USED bit.
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---
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Summary for the pthread_rwlock_tryrdlock() stall:
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The stall is caused by pthread_rwlock_tryrdlock failing to check
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that PTHREAD_RWLOCK_FUTEX_USED is set in the __wrphase_futex futex
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and then waking the futex.
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The fix for bug 23844 ensures that waiters on __wrphase_futex are
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correctly woken. Before the fix the test stalls as readers can
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wait forever on __wrphase_futex. */
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <pthread.h>
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#include <support/xthread.h>
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#include <errno.h>
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/* We need only one lock to reproduce the issue. We will need multiple
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threads to get the exact case where we have a read, try, and unlock
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all interleaving to produce the case where the readers are waiting
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and the try fails to wake them. */
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pthread_rwlock_t onelock;
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/* The number of threads is arbitrary but empirically chosen to have
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enough threads that we see the condition where waiting readers are
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not woken by a successful tryrdlock. */
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#define NTHREADS 32
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_Atomic int do_exit;
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void *
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run_loop (void *arg)
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{
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int i = 0, ret;
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while (!do_exit)
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{
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/* Arbitrarily choose if we are the writer or reader. Choose a
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high enough ratio of readers to writers to make it likely
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that readers block (and eventually are susceptable to
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stalling).
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If we are a writer, take the write lock, and then unlock.
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If we are a reader, try the lock, then lock, then unlock. */
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if ((i % 8) != 0)
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xpthread_rwlock_wrlock (&onelock);
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else
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{
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if ((ret = pthread_rwlock_tryrdlock (&onelock)) != 0)
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{
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if (ret == EBUSY)
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xpthread_rwlock_rdlock (&onelock);
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else
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exit (EXIT_FAILURE);
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}
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}
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/* Thread does some work and then unlocks. */
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xpthread_rwlock_unlock (&onelock);
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i++;
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}
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return NULL;
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}
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int
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do_test (void)
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{
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int i;
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pthread_t tids[NTHREADS];
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xpthread_rwlock_init (&onelock, NULL);
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for (i = 0; i < NTHREADS; i++)
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tids[i] = xpthread_create (NULL, run_loop, NULL);
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/* Run for some amount of time. Empirically speaking exercising
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the stall via pthread_rwlock_tryrdlock is much harder, and on
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a 3.5GHz 4 core x86_64 VM system it takes somewhere around
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20-200s to stall, approaching 100% stall past 200s. We can't
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wait that long for a regression test so we just test for 20s,
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and expect the stall to happen with a 5-10% chance (enough for
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developers to see). */
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sleep (20);
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/* Then exit. */
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printf ("INFO: Exiting...\n");
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do_exit = 1;
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/* If any readers stalled then we will timeout waiting for them. */
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for (i = 0; i < NTHREADS; i++)
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xpthread_join (tids[i]);
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printf ("INFO: Done.\n");
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xpthread_rwlock_destroy (&onelock);
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printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n");
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return 0;
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}
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#define TIMEOUT 30
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#include <support/test-driver.c>
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