[PATCH bpf-next v1 10/22] rqspinlock: Protect waiters in trylock fallback from stalls

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

When we run out of maximum rqnodes, the original queued spin lock slow
path falls back to a try lock. In such a case, we are again susceptible
to stalls in case the lock owner fails to make progress. We use the
timeout as a fallback to break out of this loop and return to the
caller. This is a fallback for an extreme edge case, when on the same
CPU we run out of all 4 qnodes. When could this happen? We are in slow
path in task context, we get interrupted by an IRQ, which while in the
slow path gets interrupted by an NMI, whcih in the slow path gets
another nested NMI, which enters the slow path. All of the interruptions
happen after node->count++.

Reviewed-by: Barret Rhoden <brho@google.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/locking/rqspinlock.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rqspinlock.c b/kernel/locking/rqspinlock.c
index f712fe4b1f38..b63f92bd43b1 100644
--- a/kernel/locking/rqspinlock.c
+++ b/kernel/locking/rqspinlock.c
@@ -255,8 +255,14 @@ int __lockfunc resilient_queued_spin_lock_slowpath(struct qspinlock *lock, u32 v
 	 */
 	if (unlikely(idx >= _Q_MAX_NODES)) {
 		lockevent_inc(lock_no_node);
-		while (!queued_spin_trylock(lock))
+		RES_RESET_TIMEOUT(ts);
+		while (!queued_spin_trylock(lock)) {
+			if (RES_CHECK_TIMEOUT(ts, ret)) {
+				lockevent_inc(rqspinlock_lock_timeout);
+				break;
+			}
 			cpu_relax();
+		}
 		goto release;
 	}
 
-- 
2.43.5