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Re: [patch] testsuite: Fix a race by me - watchthreads-reorder.exp
- From: Jan Kratochvil <jan dot kratochvil at redhat dot com>
- To: Tom Tromey <tromey at redhat dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Tue, 19 Jan 2010 23:12:28 +0100
- Subject: Re: [patch] testsuite: Fix a race by me - watchthreads-reorder.exp
- References: <20091217195026.GA21468@host0.dyn.jankratochvil.net> <m34onpgxcj.fsf@fleche.redhat.com>
On Thu, 17 Dec 2009 21:27:24 +0100, Tom Tromey wrote:
> >>>>> "Jan" == Jan Kratochvil <jan.kratochvil@redhat.com> writes:
> Jan> + i = pthread_cond_wait (&thread1_tid_cond, &thread1_tid_mutex);
> Jan> + assert (i == 0);
>
> pthread_cond_wait can also spuriously wake up. The usual thing to do is
> call it in a loop that checks some condition. Then, have the signalling
> thread set the condition before calling pthread_cond_signal. Something
> like:
>
> while (thread1_tid == 0)
> pthread_cond_wait (...);
>
> This is race-free as long as the signalling thread also acquires the
> mutex associated with the condition.
>
> Is there some reason not to do this in this test case?
Thanks, I was not proficient in these functions (and have not read the whole
manual).
OK to check-in?
Thanks,
Jan
gdb/testsuite/
2010-01-19 Jan Kratochvil <jan.kratochvil@redhat.com>
* gdb.threads/watchthreads-reorder.c (gdbstop_mutex): Remove.
(thread1_func): Protect thread1_tid with thread1_tid_cond by
thread1_tid_mutex. Remove gdbstop_mutex handling.
(thread2_func): Protect thread2_tid with thread2_tid_cond by
thread2_tid_mutex. Remove gdbstop_mutex handling.
(main): Move thread1_tid_mutex and thread2_tid_mutex locks before
pthread_create. Remove gdbstop_mutex handling. New comment. Replace
pthread_cond_wait conditionalizations by while loops.
--- a/gdb/testsuite/gdb.threads/watchthreads-reorder.c
+++ b/gdb/testsuite/gdb.threads/watchthreads-reorder.c
@@ -34,8 +34,6 @@
otherwise. */
#define TIMEOUT (gettid () == getpid() ? 10 : 15)
-static pthread_mutex_t gdbstop_mutex = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;
-
static pid_t thread1_tid;
static pthread_cond_t thread1_tid_cond = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t thread1_tid_mutex = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;
@@ -89,13 +87,14 @@ thread1_func (void *unused)
int i;
volatile int rwatch_store;
+ timed_mutex_lock (&thread1_tid_mutex);
+
+ /* THREAD1_TID_MUTEX must be already locked to avoid race. */
thread1_tid = gettid ();
+
i = pthread_cond_signal (&thread1_tid_cond);
assert (i == 0);
-
- /* Be sure GDB is already stopped before continuing. */
- timed_mutex_lock (&gdbstop_mutex);
- i = pthread_mutex_unlock (&gdbstop_mutex);
+ i = pthread_mutex_unlock (&thread1_tid_mutex);
assert (i == 0);
rwatch_store = thread1_rwatch;
@@ -114,13 +113,14 @@ thread2_func (void *unused)
int i;
volatile int rwatch_store;
+ timed_mutex_lock (&thread2_tid_mutex);
+
+ /* THREAD2_TID_MUTEX must be already locked to avoid race. */
thread2_tid = gettid ();
+
i = pthread_cond_signal (&thread2_tid_cond);
assert (i == 0);
-
- /* Be sure GDB is already stopped before continuing. */
- timed_mutex_lock (&gdbstop_mutex);
- i = pthread_mutex_unlock (&gdbstop_mutex);
+ i = pthread_mutex_unlock (&thread2_tid_mutex);
assert (i == 0);
rwatch_store = thread2_rwatch;
@@ -267,7 +267,8 @@ main (int argc, char **argv)
setbuf (stdout, NULL);
- timed_mutex_lock (&gdbstop_mutex);
+ timed_mutex_lock (&thread1_tid_mutex);
+ timed_mutex_lock (&thread2_tid_mutex);
timed_mutex_lock (&terminate_mutex);
@@ -306,29 +307,23 @@ main (int argc, char **argv)
state_wait (tracer, "T (stopped)");
}
- timed_mutex_lock (&thread1_tid_mutex);
- timed_mutex_lock (&thread2_tid_mutex);
-
- /* Let the threads start. */
- i = pthread_mutex_unlock (&gdbstop_mutex);
- assert (i == 0);
+ /* Threads are now waiting at timed_mutex_lock (thread1_tid_mutex) and so
+ they could not trigger the watchpoints before GDB gets unstopped later.
+ Threads get resumed at pthread_cond_wait below. Use `while' loops for
+ protection against spurious pthread_cond_wait wakeups. */
printf ("Waiting till the threads initialize their TIDs.\n");
- if (thread1_tid == 0)
+ while (thread1_tid == 0)
{
i = pthread_cond_wait (&thread1_tid_cond, &thread1_tid_mutex);
assert (i == 0);
-
- assert (thread1_tid > 0);
}
- if (thread2_tid == 0)
+ while (thread2_tid == 0)
{
i = pthread_cond_wait (&thread2_tid_cond, &thread2_tid_mutex);
assert (i == 0);
-
- assert (thread2_tid > 0);
}
printf ("Thread 1 TID = %lu, thread 2 TID = %lu, PID = %lu.\n",