[v2] SUNRPC: Fix a race to wake a sync task

Commit Message

Benjamin Coddington July 17, 2024, 2:49 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__after_atomic() before the wake_up_bit(),
which pairs with prepare_to_wait() calling set_current_state().

Signed-off-by: Benjamin Coddington <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
---
This v2 drops the comment which matches other barriers in our subsystem, and
backs out the full memory barrier to use the __after_atomic() variant as
Trond suggests.

Though I have not yet completed a full test run, it has survived 16 of the
50 hours I planned for it.  My reproducer rarely takes longer than an hour,
never more than 90 minutes for the last ~10 reproductions.

Thanks for the review and attention.
---
 net/sunrpc/sched.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Patch

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 6debf4fd42d4..cef623ea1506 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -369,8 +369,10 @@  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 {
+		smp_mb__after_atomic();
 		wake_up_bit(&task->tk_runstate, RPC_TASK_QUEUED);
+	}
 }
 
 /*