[PATCH v3 3/3] test-ww_mutex: Handle transient -EDEADLK in test_cycle_work

["Håkon Bugge" <haakon.bugge@oracle.com>]

There is a timing issue in test_cycle_work(), in the sense that
acquiring *a_mutex* after deadlock has been detected on the *b_mutex*,
may not succeed immediately. This may lead to false negatives, which
shows up in the log as:

   cyclic deadlock not resolved, ret[77/93] = -35

We re-factor the inner part test_cycle_work(), where we loop a few
times attempting to acquire the mutexes in BAAB order. If the first A
succeeds or the last B succeeds, we break out of the loop.

Fixes: d1b42b800e5d ("locking/ww_mutex: Add kselftests for resolving ww_mutex cyclic deadlocks")
Fixes: e4a02ed2aaf4 ("locking/ww_mutex: Fix runtime warning in the WW mutex selftest")
Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Acked-by: John Stultz <jstultz@google.com>

---

v2 -> v3:
      * Added John's a-b
      * Renamed the "ok" label to "out", as suggested by John

v1 -> v2:
      * Removed the end-less lock acquisition attempts, as that
        strategy did not obey the big-rules of the WW
        mutexes. Instead, re-factor the test, and loop a handful
        times.
---
 kernel/locking/test-ww_mutex.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/test-ww_mutex.c b/kernel/locking/test-ww_mutex.c
index 5ed91f2cf30d9..47e016a4f4fea 100644
--- a/kernel/locking/test-ww_mutex.c
+++ b/kernel/locking/test-ww_mutex.c
@@ -294,6 +294,8 @@ static void test_cycle_work(struct work_struct *work)
 	struct test_cycle *cycle = container_of(work, typeof(*cycle), work);
 	struct ww_acquire_ctx ctx;
 	int err, erra = 0;
+	const int max_attempts = 5;
+	int attempt;
 
 	ww_acquire_init_noinject(&ctx, cycle->class);
 	ww_mutex_lock(&cycle->a_mutex, &ctx);
@@ -302,13 +304,26 @@ static void test_cycle_work(struct work_struct *work)
 	wait_for_completion(&cycle->b_signal);
 
 	err = ww_mutex_lock(cycle->b_mutex, &ctx);
-	if (err == -EDEADLK) {
+	if (err != -EDEADLK)
+		goto out;
+
+	for (attempt = 0; attempt < max_attempts; ++attempt) {
 		err = 0;
 		ww_mutex_unlock(&cycle->a_mutex);
 		ww_mutex_lock_slow(cycle->b_mutex, &ctx);
 		erra = ww_mutex_lock(&cycle->a_mutex, &ctx);
+		if (erra != -EDEADLK)
+			break;
+
+		erra = 0;
+		ww_mutex_unlock(cycle->b_mutex);
+		ww_mutex_lock_slow(&cycle->a_mutex, &ctx);
+		err = ww_mutex_lock(cycle->b_mutex, &ctx);
+		if (err != -EDEADLK)
+			break;
 	}
 
+out:
 	if (!err)
 		ww_mutex_unlock(cycle->b_mutex);
 	if (!erra)
-- 
2.43.5