From patchwork Tue Jan 22 18:12:08 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Carlos O'Donell X-Patchwork-Id: 31162 Received: (qmail 22931 invoked by alias); 22 Jan 2019 18:12:18 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 22921 invoked by uid 89); 22 Jan 2019 18:12:18 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-26.9 required=5.0 tests=BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, KAM_SHORT, RCVD_IN_DNSWL_NONE autolearn=ham version=3.3.2 spammy=becoming, 440 X-HELO: mail-qt1-f176.google.com Return-Path: Subject: [PATCHv2] nptl: Fix pthread_rwlock_try*lock stalls [BZ #23844] To: Florian Weimer Cc: Torvald Riegel , Rik Prohaska , GNU C Library , Siddhesh Poyarekar References: <878szcevu9.fsf@oldenburg2.str.redhat.com> From: Carlos O'Donell Openpgp: preference=signencrypt Message-ID: <390f7db7-8e72-e718-f0eb-673eb91199c0@redhat.com> Date: Tue, 22 Jan 2019 13:12:08 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <878szcevu9.fsf@oldenburg2.str.redhat.com> On 1/22/19 7:10 AM, Florian Weimer wrote: > * Carlos O'Donell: > >> +/* For a full analysis see comment: >> + https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14 > > In the past, we had issues with bug trackers going away and such > references becoming stale. Fixed. Added text to first test. >> + >> + 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 >> +#include >> +#include >> +#include >> +#include >> +#include >> + >> +/* 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, rw; >> + 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). */ >> + rw = i % 8; >> + /* 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 (rw) > > rw != 0 (or (i % 8) != 0). Note sure if the variable actually increases > clarity here. Fixed. I'll remove rw and avoid boolean coercion. >> + /* 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); > > Use nanosleep or usleep to avoid signals. Not fixed. I'm going to leave this as sleep for now. I'm going to change this to xnanosleep after release. I'll add a support/xnanosleep.c with a nanosleep that checks for errors. I will also fixup the new tst-rwloc-pwn.c test too which uses sleep. >> + /* Then exit. */ >> + printf ("INFO: Exiting..."); >> + fflush (stdout); > > Why no trailing \n? It will just result in garbled output if there is a > problem. Fixed. Because I wanted a nice "Exiting...done" message. It matches the "done." later in the output which has a \n. I'll fix this up though, you raise a good design point, that all messages should immediately get output and be seen, rather than delay them. It's simpler to just use: printf ("INFO: Exiting...\n"); ... printf ("INFO: Done.\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 ("done.\n"); >> + xpthread_rwlock_destroy (&onelock); >> + printf ("PASS: No pthread_rwlock_tryrdlock stalls detected.\n"); >> + return 0; >> +} >> + >> +#define TIMEOUT (30) > > Parentheses are unnecessary. Fixed. > Most of these comments also apply to ther other test. Fixed in both places. > I have not reviewed the code. > > Thanks, > Florian > v2 - Added analysis comment to tryrdlock test. - Removed rw var and avoided boolean coercion. - Avoided incomplete test output to stdout. From 0768c51e2930f922071b4b77f96bb15d1ddae004 Mon Sep 17 00:00:00 2001 From: Carlos O'Donell Date: Mon, 21 Jan 2019 22:50:12 -0500 Subject: [PATCH] nptl: Fix pthread_rwlock_try*lock stalls (Bug 23844) For a full analysis of both the pthread_rwlock_tryrdlock() stall and the pthread_rwlock_trywrlock() stall see: https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14 In the pthread_rwlock_trydlock() function we fail to inspect for PTHREAD_RWLOCK_FUTEX_USED in __wrphase_futex and wake the waiting readers. In the pthread_rwlock_trywrlock() function we write 1 to __wrphase_futex and loose the setting of the PTHREAD_RWLOCK_FUTEX_USED bit, again failing to wake waiting readers during unlock. The fix in the case of pthread_rwlock_trydlock() is to check for PTHREAD_RWLOCK_FUTEX_USED and wake the readers. The fix in the case of pthread_rwlock_trywrlock() is to only write 1 to __wrphase_futex if we installed the write phase, since all other readers would be spinning waiting for this step. We add two new tests, one exercises the stall for pthread_rwlock_trywrlock() which is easy to exercise, and one exercises the stall for pthread_rwlock_trydlock() which is harder to exercise. The pthread_rwlock_trywrlock() test fails consistently without the fix, and passes after. The pthread_rwlock_tryrdlock() test fails roughly 5-10% of the time without the fix, and passes all the time after. Signed-off-by: Carlos O'Donell Signed-off-by: Torvald Riegel Signed-off-by: Rik Prohaska --- ChangeLog | 17 ++ nptl/Makefile | 3 +- nptl/pthread_rwlock_tryrdlock.c | 25 ++- nptl/pthread_rwlock_trywrlock.c | 9 +- nptl/tst-rwlock-tryrdlock-stall.c | 355 ++++++++++++++++++++++++++++++ nptl/tst-rwlock-trywrlock-stall.c | 108 +++++++++ support/Makefile | 1 + support/xpthread_rwlock_destroy.c | 26 +++ support/xthread.h | 1 + 9 files changed, 534 insertions(+), 11 deletions(-) create mode 100644 nptl/tst-rwlock-tryrdlock-stall.c create mode 100644 nptl/tst-rwlock-trywrlock-stall.c create mode 100644 support/xpthread_rwlock_destroy.c diff --git a/ChangeLog b/ChangeLog index c4fc9ab22a..c8eb7a397d 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,20 @@ +2019-01-22 Carlos O'Donell + Torvald Riegel + Rik Prohaska + + [BZ# 23844] + * nptl/Makefile (tests): Add tst-rwlock-tryrdlock-stall, and + tst-rwlock-trywrlock-stall. + * nptl/pthread_rwlock_tryrdlock.c (__pthread_rwlock_tryrdlock): + Wake waiters if PTHREAD_RWLOCK_FUTEX_USED is set. + * nptl/pthread_rwlock_trywrlock.c (__pthread_rwlock_trywrlock): + Set __wrphase_fute to 1 only if we started the write phase. + * nptl/tst-rwlock-tryrdlock-stall.c: New file. + * nptl/tst-rwlock-trywrlock-stall.c: New file. + * support/Makefile (libsupport-routines): Add xpthread_rwlock_destroy. + * support/xpthread_rwlock_destroy.c: New file. + * support/xthread.h: Declare xpthread_rwlock_destroy. + 2019-01-21 Joseph Myers * scripts/build-many-glibcs.py (Context.checkout): Default diff --git a/nptl/Makefile b/nptl/Makefile index 340282c6cb..0e316edfac 100644 --- a/nptl/Makefile +++ b/nptl/Makefile @@ -319,7 +319,8 @@ tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \ tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \ tst-cnd-timedwait tst-thrd-detach tst-mtx-basic tst-thrd-sleep \ tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock \ - tst-rwlock-pwn + tst-rwlock-pwn \ + tst-rwlock-tryrdlock-stall tst-rwlock-trywrlock-stall tests-internal := tst-rwlock19 tst-rwlock20 \ tst-sem11 tst-sem12 tst-sem13 \ diff --git a/nptl/pthread_rwlock_tryrdlock.c b/nptl/pthread_rwlock_tryrdlock.c index 368862ff07..2f94f17f36 100644 --- a/nptl/pthread_rwlock_tryrdlock.c +++ b/nptl/pthread_rwlock_tryrdlock.c @@ -94,15 +94,22 @@ __pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock) /* Same as in __pthread_rwlock_rdlock_full: We started the read phase, so we are also responsible for updating the write-phase futex. Relaxed MO is sufficient. - Note that there can be no other reader that we have to wake - because all other readers will see the read phase started by us - (or they will try to start it themselves); if a writer started - the read phase, we cannot have started it. Furthermore, we - cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will - overwrite the value set by the most recent writer (or the readers - before it in case of explicit hand-over) and we know that there - are no waiting readers. */ - atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0); + We have to do the same steps as a writer would when handing over the + read phase to use because other readers cannot distinguish between + us and the writer. + Note that __pthread_rwlock_tryrdlock callers will not have to be + woken up because they will either see the read phase started by us + or they will try to start it themselves; however, callers of + __pthread_rwlock_rdlock_full just increase the reader count and then + check what state the lock is in, so they cannot distinguish between + us and a writer that acquired and released the lock in the + meantime. */ + if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0) + & PTHREAD_RWLOCK_FUTEX_USED) != 0) + { + int private = __pthread_rwlock_get_private (rwlock); + futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private); + } } return 0; diff --git a/nptl/pthread_rwlock_trywrlock.c b/nptl/pthread_rwlock_trywrlock.c index fd37a71ce4..fae475cc70 100644 --- a/nptl/pthread_rwlock_trywrlock.c +++ b/nptl/pthread_rwlock_trywrlock.c @@ -46,8 +46,15 @@ __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock) &rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED)) { + /* We have become the primary writer and we cannot have shared + the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we + can simply enable blocking (see full wrlock code). */ atomic_store_relaxed (&rwlock->__data.__writers_futex, 1); - atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); + /* If we started a write phase, we need to enable readers to + wait. If we did not, we must not change it because other threads + may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime. */ + if ((r & PTHREAD_RWLOCK_WRPHASE) == 0) + atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); atomic_store_relaxed (&rwlock->__data.__cur_writer, THREAD_GETMEM (THREAD_SELF, tid)); return 0; diff --git a/nptl/tst-rwlock-tryrdlock-stall.c b/nptl/tst-rwlock-tryrdlock-stall.c new file mode 100644 index 0000000000..9351ad3338 --- /dev/null +++ b/nptl/tst-rwlock-tryrdlock-stall.c @@ -0,0 +1,355 @@ +/* Bug 23844: Test for pthread_rwlock_tryrdlock stalls. + Copyright (C) 2019 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 + . */ + +/* 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 +#include +#include +#include +#include +#include + +/* 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 diff --git a/nptl/tst-rwlock-trywrlock-stall.c b/nptl/tst-rwlock-trywrlock-stall.c new file mode 100644 index 0000000000..14d27cbcbc --- /dev/null +++ b/nptl/tst-rwlock-trywrlock-stall.c @@ -0,0 +1,108 @@ +/* Bug 23844: Test for pthread_rwlock_trywrlock stalls. + Copyright (C) 2019 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 + . */ + +/* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c. + + Summary for the pthread_rwlock_trywrlock() stall: + + The stall is caused by pthread_rwlock_trywrlock setting + __wrphase_futex futex to 1 and loosing the + PTHREAD_RWLOCK_FUTEX_USED bit. + + 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 +#include +#include +#include +#include +#include + +/* 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 clears the PTHREAD_RWLOCK_FUTEX_USED bit and a + subsequent unlock 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 unlock. */ +#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) + { + if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0) + { + if (ret == EBUSY) + xpthread_rwlock_wrlock (&onelock); + else + exit (EXIT_FAILURE); + } + } + else + xpthread_rwlock_rdlock (&onelock); + /* 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. The pthread_rwlock_tryrwlock stall + is very easy to trigger and happens in seconds under the test + conditions. */ + sleep (10); + /* 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_tryrwlock stalls detected.\n"); + return 0; +} + +#include diff --git a/support/Makefile b/support/Makefile index 432cf2fe6c..c15b93647c 100644 --- a/support/Makefile +++ b/support/Makefile @@ -129,6 +129,7 @@ libsupport-routines = \ xpthread_mutexattr_settype \ xpthread_once \ xpthread_rwlock_init \ + xpthread_rwlock_destroy \ xpthread_rwlock_rdlock \ xpthread_rwlock_unlock \ xpthread_rwlock_wrlock \ diff --git a/support/xpthread_rwlock_destroy.c b/support/xpthread_rwlock_destroy.c new file mode 100644 index 0000000000..6d6e953569 --- /dev/null +++ b/support/xpthread_rwlock_destroy.c @@ -0,0 +1,26 @@ +/* pthread_rwlock_destroy with error checking. + Copyright (C) 2019 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 + . */ + +#include + +void +xpthread_rwlock_destroy (pthread_rwlock_t *rwlock) +{ + xpthread_check_return ("pthread_rwlock_destroy", + pthread_rwlock_destroy (rwlock)); +} diff --git a/support/xthread.h b/support/xthread.h index 47c23235f3..9fe1f68b3b 100644 --- a/support/xthread.h +++ b/support/xthread.h @@ -84,6 +84,7 @@ void xpthread_rwlockattr_setkind_np (pthread_rwlockattr_t *attr, int pref); void xpthread_rwlock_wrlock (pthread_rwlock_t *rwlock); void xpthread_rwlock_rdlock (pthread_rwlock_t *rwlock); void xpthread_rwlock_unlock (pthread_rwlock_t *rwlock); +void xpthread_rwlock_destroy (pthread_rwlock_t *rwlock); __END_DECLS