[PATCH bpf-next v4 11/25] 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++.

We use RES_DEF_TIMEOUT as our spinning duration, but in the case of this
fallback, no fairness is guaranteed, so the duration may be too small
for contended cases, as the waiting time is not bounded. Since this is
an extreme corner case, let's just prefer timing out instead of
attempting to spin for longer.

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

diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
index 65c2b41d8937..361d452f027c 100644
--- a/kernel/bpf/rqspinlock.c
+++ b/kernel/bpf/rqspinlock.c
@@ -275,8 +275,14 @@ 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, RES_DEF_TIMEOUT);
+		while (!queued_spin_trylock(lock)) {
+			if (RES_CHECK_TIMEOUT(ts, ret)) {
+				lockevent_inc(rqspinlock_lock_timeout);
+				break;
+			}
 			cpu_relax();
+		}
 		goto release;
 	}
 
-- 
2.47.1