[PATCH bpf-next v2 10/26] 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 fdc20157d0c9..df7adec59cec 100644
--- a/kernel/locking/rqspinlock.c
+++ b/kernel/locking/rqspinlock.c
@@ -255,8 +255,14 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val,
 	 */
 	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