diff mbox series

[RFC,1/1] SUNRPC: Fix a race to wake a sync task

Message ID 4c1c7cd404173b9888464a2d7e2a430cc7e18737.1720643005.git.bcodding@redhat.com (mailing list archive)
State New
Headers show
Series A rare failure to wake up sync tasks | expand

Commit Message

Benjamin Coddington July 10, 2024, 8:32 p.m. UTC
We've observed NFS clients with sync tasks sleeping in __rpc_execute
waiting on RPC_TASK_QUEUED that have not responded to a wake-up from
rpc_make_runnable().  I suspect this problem usually goes unnoticed,
because on a busy client the task will eventually be re-awoken by another
task completion or xprt event.  However, if the state manager is draining
the slot table, a sync task missing a wake-up can result in a hung client.

We've been able to prove that the waker in rpc_make_runnable() successfully
calls wake_up_bit() (ie- there's no race to tk_runstate), but the
wake_up_bit() call fails to wake the waiter.  I suspect the waker is
missing the load of the bit's wait_queue_head, so waitqueue_active() is
false.  There are some very helpful comments about this problem above
wake_up_bit(), prepare_to_wait(), and waitqueue_active().

Fix this by inserting smp_mb() before the wake_up_bit(), which pairs with
prepare_to_wait() calling set_current_state().

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 net/sunrpc/sched.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
diff mbox series

Patch

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 6debf4fd42d4..34b31be75497 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -369,8 +369,11 @@  static void rpc_make_runnable(struct workqueue_struct *wq,
 	if (RPC_IS_ASYNC(task)) {
 		INIT_WORK(&task->u.tk_work, rpc_async_schedule);
 		queue_work(wq, &task->u.tk_work);
-	} else
+	} else {
+		/* paired with set_current_state() in prepare_to_wait */
+		smp_mb();
 		wake_up_bit(&task->tk_runstate, RPC_TASK_QUEUED);
+	}
 }
 
 /*