prio-wake: avoid glibc to kernel sleep race

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 <[email protected]>

Acked-by Will Schmidt <[email protected]>

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

Index: realtime/func/prio-wake/prio-wake.c
===================================================================

func/prio-wake/prio-wake.c +4 -6

@@ -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;
 }
 

This patch does not appear in current CVS. Please apply.

Thanks,

Darren Hart

Quoted 57 lines · Darren Hart

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 <[email protected]>

Acked-by Will Schmidt <[email protected]>

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

Index: realtime/func/prio-wake/prio-wake.c
===================================================================

func/prio-wake/prio-wake.c +4 -6

@@ -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;
 }
 

Signature · 9 lines

Darren Hart
IBM Linux Technology Center
Real-Time Linux Team

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