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List:       ltp-list
Subject:    [LTP] [PATCH] prio-wake: avoid glibc to kernel sleep race
From:       Darren Hart <dvhltc () us ! ibm ! com>
Date:       2010-01-28 19:39:35
Message-ID: 4B61E7F7.3000502 () us ! ibm ! com
[Download RAW message or body]

prio-wake: avoid glibc to kernel sleep race

In the unlocked broadcast scenario, there exists a race between when the
running_threads variable reaches rt_threads and when the last worker_thread
blocks in the kernel after a cond_wait(). It is possible for a thread to miss
the broadcast if it fails to sleep before the broadcast is issued.  

The previous code did not guarantee a small window of time to allow the threads
to get to sleep.  It also used an unreasonably large sleep time which
unnecessarily extended the length of the test run time.  This patch ensures some
time is given to the threads to get to sleep and at the same time uses a much
shorter (1000x) sleep period which results in a 50-100x reduction in test run
time. Lastly, two unecessary loops waiting for the threads to complete were
removed, relying on pthread_join() instead to wait for the threads to complete.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Will Schmidt <will_schmidt@vnet.ibm.com>

---
 func/prio-wake/prio-wake.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

Index: realtime/func/prio-wake/prio-wake.c
===================================================================
--- realtime.orig/func/prio-wake/prio-wake.c
+++ realtime/func/prio-wake/prio-wake.c
@@ -108,7 +108,10 @@ void *master_thread(void* arg)
 
 	/* make sure children are started */
 	while (running_threads < rt_threads)
-		sleep(1);
+		usleep(1000);
+	/* give the worker threads a chance to get to sleep in the kernel
+	 * in the unlocked broadcast case. */
+	usleep(1000);
 
 	start = rt_gettime() - beginrun;
 
@@ -120,8 +123,6 @@ void *master_thread(void* arg)
 	if (locked_broadcast)
 		rc = pthread_mutex_unlock(&mutex);
 
-	while (running_threads > 0)
-		sleep(1);
 	return NULL;
 }
 
@@ -157,9 +158,6 @@ void *worker_thread(void* arg)
 
 	rc = pthread_mutex_unlock(&mutex);
 
-	/* wait for all threads to quit */
-	while (running_threads > 0)
-		sleep(1);
 	return NULL;
 }
 
-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team

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