135 lines
4.7 KiB
Diff
135 lines
4.7 KiB
Diff
commit f21e8f8ca466320fed38bdb71526c574dae98026
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Author: Andreas Schwab <schwab@suse.de>
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Date: Thu Nov 8 14:28:22 2018 +0100
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Fix rwlock stall with PREFER_WRITER_NONRECURSIVE_NP (bug 23861)
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In the read lock function (__pthread_rwlock_rdlock_full) there was a
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code path which would fail to reload __readers while waiting for
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PTHREAD_RWLOCK_RWAITING to change. This failure to reload __readers
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into a local value meant that various conditionals used the old value
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of __readers and with only two threads left it could result in an
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indefinite stall of one of the readers (waiting for PTHREAD_RWLOCK_RWAITING
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to go to zero, but it never would).
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diff --git a/nptl/Makefile b/nptl/Makefile
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index ee720960d18f33d1..2d2db648f730db61 100644
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--- a/nptl/Makefile
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+++ b/nptl/Makefile
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@@ -318,7 +318,8 @@ tests = tst-attr1 tst-attr2 tst-attr3 tst-default-attr \
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tst-minstack-throw \
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tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \
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tst-cnd-timedwait tst-thrd-detach tst-mtx-basic tst-thrd-sleep \
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- tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock
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+ tst-mtx-recursive tst-tss-basic tst-call-once tst-mtx-timedlock \
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+ tst-rwlock-pwn
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tests-internal := tst-rwlock19 tst-rwlock20 \
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tst-sem11 tst-sem12 tst-sem13 \
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diff --git a/nptl/pthread_rwlock_common.c b/nptl/pthread_rwlock_common.c
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index 5dd534271aed6b41..85fc1bcfc7f5e60d 100644
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--- a/nptl/pthread_rwlock_common.c
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+++ b/nptl/pthread_rwlock_common.c
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@@ -314,7 +314,7 @@ __pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
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harmless because the flag is just about the state of
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__readers, and all threads set the flag under the same
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conditions. */
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- while ((atomic_load_relaxed (&rwlock->__data.__readers)
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+ while (((r = atomic_load_relaxed (&rwlock->__data.__readers))
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& PTHREAD_RWLOCK_RWAITING) != 0)
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{
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int private = __pthread_rwlock_get_private (rwlock);
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diff --git a/nptl/tst-rwlock-pwn.c b/nptl/tst-rwlock-pwn.c
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new file mode 100644
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index 0000000000000000..c39dd70973f1a76e
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--- /dev/null
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+++ b/nptl/tst-rwlock-pwn.c
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@@ -0,0 +1,87 @@
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+/* Test rwlock with PREFER_WRITER_NONRECURSIVE_NP (bug 23861).
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+ Copyright (C) 2018 Free Software Foundation, Inc.
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+ This file is part of the GNU C Library.
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+
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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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+
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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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+
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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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+ <http://www.gnu.org/licenses/>. */
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+
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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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+
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+/* We choose 10 iterations because this happens to be able to trigger the
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+ stall on contemporary hardware. */
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+#define LOOPS 10
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+/* We need 3 threads to trigger bug 23861. One thread as a writer, and
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+ two reader threads. The test verifies that the second-to-last reader
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+ is able to notify the *last* reader that it should be done waiting.
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+ If the second-to-last reader fails to notify the last reader or does
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+ so incorrectly then the last reader may stall indefinitely. */
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+#define NTHREADS 3
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+
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+_Atomic int do_exit;
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+pthread_rwlockattr_t mylock_attr;
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+pthread_rwlock_t mylock;
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+
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+void *
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+run_loop (void *a)
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+{
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+ while (!do_exit)
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+ {
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+ if (random () & 1)
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+ {
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+ xpthread_rwlock_wrlock (&mylock);
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+ xpthread_rwlock_unlock (&mylock);
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+ }
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+ else
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+ {
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+ xpthread_rwlock_rdlock (&mylock);
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+ xpthread_rwlock_unlock (&mylock);
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+ }
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+ }
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+ return NULL;
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+}
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+
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+int
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+do_test (void)
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+{
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+ xpthread_rwlockattr_init (&mylock_attr);
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+ xpthread_rwlockattr_setkind_np (&mylock_attr,
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+ PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
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+ xpthread_rwlock_init (&mylock, &mylock_attr);
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+
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+ for (int n = 0; n < LOOPS; n++)
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+ {
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+ pthread_t tids[NTHREADS];
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+ do_exit = 0;
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+ for (int i = 0; i < NTHREADS; i++)
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+ tids[i] = xpthread_create (NULL, run_loop, NULL);
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+ /* Let the threads run for some time. */
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+ sleep (1);
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+ printf ("Exiting...");
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+ fflush (stdout);
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+ do_exit = 1;
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+ for (int i = 0; i < NTHREADS; i++)
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+ xpthread_join (tids[i]);
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+ printf ("done.\n");
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+ }
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+ pthread_rwlock_destroy (&mylock);
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+ pthread_rwlockattr_destroy (&mylock_attr);
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+ return 0;
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+}
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+
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+#define TIMEOUT (DEFAULT_TIMEOUT + 3 * LOOPS)
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+#include <support/test-driver.c>
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