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[PATCH] Fix lost wake-up when pthread_rwlock_timedrwlock times out.
- From: Torvald Riegel <triegel at redhat dot com>
- To: GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Wed, 22 Apr 2015 17:06:49 +0200
- Subject: [PATCH] Fix lost wake-up when pthread_rwlock_timedrwlock times out.
- Authentication-results: sourceware.org; auth=none
If we set up a rwlock to prefer writers (and disallow recursive rdlock
acquisitions), then readers will block for writers that are blocked to
acquire the lock (otherwise, readers could constantly enter and exit,
and the writer would never get the lock). However, the existing
implementation did not wake such readers when the writer timed out.
This patch adds the missing wake-up.
There's no similar case for writers being blocked on readers.
Tested on x86_64-linux. OK?
2015-04-22 Torvald Riegel <triegel@redhat.com>
* nptl/pthread_rwlock_timedwrlock.c (pthread_rwlock_timedwrlock): Add
missing wake-up of readers.
* nptl/tst-rwlock15.c: New file.
* nptl/Makefile (tests): Add new test.
commit cd5026a0376eaba294e41a18f21564b27b11c7e8
Author: Torvald Riegel <triegel@redhat.com>
Date: Tue Apr 21 20:34:21 2015 +0200
Fix lost wake-up when pthread_rwlock_timedrwlock times out.
diff --git a/nptl/Makefile b/nptl/Makefile
index d784c8d..7e1897c 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -224,6 +224,7 @@ tests = tst-typesizes \
tst-rwlock1 tst-rwlock2 tst-rwlock2a tst-rwlock3 tst-rwlock4 \
tst-rwlock5 tst-rwlock6 tst-rwlock7 tst-rwlock8 tst-rwlock9 \
tst-rwlock10 tst-rwlock11 tst-rwlock12 tst-rwlock13 tst-rwlock14 \
+ tst-rwlock15 \
tst-once1 tst-once2 tst-once3 tst-once4 \
tst-key1 tst-key2 tst-key3 tst-key4 \
tst-sem1 tst-sem2 tst-sem3 tst-sem4 tst-sem5 tst-sem6 tst-sem7 \
diff --git a/nptl/pthread_rwlock_timedwrlock.c b/nptl/pthread_rwlock_timedwrlock.c
index 416f6e2..58464b4 100644
--- a/nptl/pthread_rwlock_timedwrlock.c
+++ b/nptl/pthread_rwlock_timedwrlock.c
@@ -32,6 +32,7 @@ pthread_rwlock_timedwrlock (rwlock, abstime)
const struct timespec *abstime;
{
int result = 0;
+ int wake_readers = 0;
/* Make sure we are alone. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
@@ -136,6 +137,16 @@ pthread_rwlock_timedwrlock (rwlock, abstime)
if (err == -ETIMEDOUT)
{
result = ETIMEDOUT;
+ /* If we prefer writers, it can have happened that readers blocked
+ for us to acquire the lock first. If we have timed out, we need
+ to wake such readers if there are any (using the same steps we
+ would do during a normal unlock). */
+ if (!PTHREAD_RWLOCK_PREFER_READER_P (rwlock)
+ && (rwlock->__data.__nr_readers_queued > 0))
+ {
+ ++rwlock->__data.__readers_wakeup;
+ wake_readers = 1;
+ }
break;
}
}
@@ -143,5 +154,10 @@ pthread_rwlock_timedwrlock (rwlock, abstime)
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
+ /* Might be required after timeouts. */
+ if (wake_readers)
+ lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
+ rwlock->__data.__shared);
+
return result;
}
diff --git a/nptl/tst-rwlock15.c b/nptl/tst-rwlock15.c
new file mode 100644
index 0000000..b292701
--- /dev/null
+++ b/nptl/tst-rwlock15.c
@@ -0,0 +1,116 @@
+/* Copyright (C) 2015 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
+ <http://www.gnu.org/licenses/>. */
+
+/* This tests that a writer that is preferred -- but times out due to a
+ reader being present -- does not miss to wake other readers blocked on the
+ writer's pending lock acquisition. */
+
+#include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+/* The bug existed in the code that strictly prefers writers over readers. */
+static pthread_rwlock_t r = PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP;
+
+static void *
+writer (void *arg)
+{
+ struct timespec ts;
+ if (clock_gettime (CLOCK_REALTIME, &ts) != 0)
+ {
+ puts ("clock_gettime failed");
+ exit (EXIT_FAILURE);
+ }
+ ts.tv_sec += 1;
+ int e = pthread_rwlock_timedwrlock (&r, &ts);
+ if (e != ETIMEDOUT)
+ {
+ puts ("timedwrlock did not time out");
+ exit (EXIT_FAILURE);
+ }
+ return NULL;
+}
+
+static void *
+reader (void *arg)
+{
+ /* This isn't a reliable way to get the interleaving we need (because a
+ failed trylock doesn't synchronize with the writer, and because we could
+ try to lock after the writer has already timed out). However, both will
+ just lead to false positives. */
+ int e;
+ while ((e = pthread_rwlock_tryrdlock (&r)) != EBUSY)
+ {
+ if (e != 0)
+ exit (EXIT_FAILURE);
+ pthread_rwlock_unlock (&r);
+ }
+ e = pthread_rwlock_rdlock (&r);
+ if (e != 0)
+ {
+ puts ("reader rdlock failed");
+ exit (EXIT_FAILURE);
+ }
+ pthread_rwlock_unlock (&r);
+ return NULL;
+}
+
+
+static int
+do_test (void)
+{
+ /* Grab a rdlock, then create a writer and a reader, and wait until they
+ finished. */
+
+ if (pthread_rwlock_rdlock (&r) != 0)
+ {
+ puts ("initial rdlock failed");
+ return 1;
+ }
+
+ pthread_t thw;
+ if (pthread_create (&thw, NULL, writer, NULL) != 0)
+ {
+ puts ("create failed");
+ return 1;
+ }
+ pthread_t thr;
+ if (pthread_create (&thr, NULL, reader, NULL) != 0)
+ {
+ puts ("create failed");
+ return 1;
+ }
+
+ if (pthread_join (thw, NULL) != 0)
+ {
+ puts ("writer join failed");
+ return 1;
+ }
+ if (pthread_join (thr, NULL) != 0)
+ {
+ puts ("reader join failed");
+ return 1;
+ }
+
+ return 0;
+}
+
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"