splat linux:shrinker: Fix race condition

Ensure the test thread blocks until the shrinker has completed its
work.  This is done by putting the test thread to sleep and waking
it each time the shrinker callback runs.  Once the shrinker size
drops to zero or we time out the test is allowed to proceed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #96
Closes #125
Closes #182

module/splat/splat-linux.c

@@ -86,6 +86,12 @@ splat_linux_test2(struct file *file, void *arg)
 	return 0;
 }
 
+/*
+ * Wait queue used to eliminate race between dropping of slab
+ * and execution of the shrinker callback
+ */
+DECLARE_WAIT_QUEUE_HEAD(shrinker_wait);
+
 SPL_SHRINKER_CALLBACK_FWD_DECLARE(splat_linux_shrinker_fn);
 SPL_SHRINKER_DECLARE(splat_linux_shrinker, splat_linux_shrinker_fn, 1);
 static unsigned long splat_linux_shrinker_size = 0;
@@ -116,6 +122,9 @@ __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 		return -1;
 	}
 
+	/* Shrinker has run, so signal back to test. */
+	wake_up(&shrinker_wait);
+
 	return (int)splat_linux_shrinker_size;
 }
 
@@ -183,7 +192,24 @@ splat_linux_test3(struct file *file, void *arg)
 	if (rc)
 		goto out;
 
-	if (splat_linux_shrinker_size != 0) {
+	/*
+	 * By the time we get here, it is possible that the shrinker has not
+	 * yet run. splat_linux_drop_slab sends a signal for it to run, but
+	 * there is no guarantee of when it will actually run. We wait for it
+	 * to run here, terminating when either the shrinker size is now 0 or
+	 * we timeout after 1 second, which should be an eternity (error).
+	 */
+	rc = wait_event_timeout(shrinker_wait, !splat_linux_shrinker_size, HZ);
+	if (!rc) {
+		splat_vprint(file, SPLAT_LINUX_TEST3_NAME,
+	            "Failed cache shrinking timed out, size now %lu",
+		    splat_linux_shrinker_size);
+		rc = -ETIMEDOUT;
+	} else {
+		rc = 0;
+	}
+
+	if (!rc && splat_linux_shrinker_size != 0) {
 		splat_vprint(file, SPLAT_LINUX_TEST3_NAME,
 	            "Failed cache was not shrunk to 0, size now %lu",
 		    splat_linux_shrinker_size);