Patch "sunrpc: fix write space race causing stalls" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

sunrpc: fix write space race causing stalls

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sunrpc-fix-write-space-race-causing-stalls.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Vrabel <[email protected]>
Date: Mon, 19 Sep 2016 13:58:30 +0100
Subject: sunrpc: fix write space race causing stalls

From: David Vrabel <[email protected]>

commit d48f9ce73c997573e1b512893fa6eddf353a6f69 upstream.

Write space becoming available may race with putting the task to sleep in xprt_wait_for_buffer_space(). The existing mechanism to avoid the race does not work.

This (edited) partial trace illustrates the problem:

[1] rpc_task_run_action: task:43546@5 ... action=call_transmit
[2] xs_write_space <-xs_tcp_write_space
[3] xprt_write_space <-xs_write_space
[4] rpc_task_sleep: task:43546@5 ...
[5] xs_write_space <-xs_tcp_write_space

[1] Task 43546 runs but is out of write space.

[2] Space becomes available, xs_write_space() clears the SOCKWQ_ASYNC_NOSPACE bit.

[3] xprt_write_space() attemts to wake xprt->snd_task (== 43546), but this has not yet been queued and the wake up is lost.

[4] xs_nospace() is called which calls xprt_wait_for_buffer_space() which queues task 43546.

[5] The call to sk->sk_write_space() at the end of xs_nospace() (which is supposed to handle the above race) does not call xprt_write_space() as the SOCKWQ_ASYNC_NOSPACE bit is clear and thus the task is not woken.

Fix the race by resetting the SOCKWQ_ASYNC_NOSPACE bit in xs_nospace() so the second call to sk->sk_write_space() calls xprt_write_space().

---
 net/sunrpc/xprtsock.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

@@ -473,7 +473,16 @@ static int xs_nospace(struct rpc_task *t
 	spin_unlock_bh(&xprt->transport_lock);
 
 	/* Race breaker in case memory is freed before above code is called */
-	sk->sk_write_space(sk);
+	if (ret == -EAGAIN) {
+		struct socket_wq *wq;
+
+		rcu_read_lock();
+		wq = rcu_dereference(sk->sk_wq);
+		set_bit(SOCKWQ_ASYNC_NOSPACE, &wq->flags);
+		rcu_read_unlock();
+
+		sk->sk_write_space(sk);
+	}
 	return ret;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/sunrpc-fix-write-space-race-causing-stalls.patch