From d48f9ce73c997573e1b512893fa6eddf353a6f69 Mon Sep 17 00:00:00 2001 From: David Vrabel Date: Mon, 19 Sep 2016 13:58:30 +0100 Subject: [PATCH] sunrpc: fix write space race causing stalls 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(). Suggested-by: Trond Myklebust Signed-off-by: David Vrabel cc: stable@vger.kernel.org # 4.4 Signed-off-by: Anna Schumaker --- net/sunrpc/xprtsock.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c index bde39f2..0137af1 100644 --- a/net/sunrpc/xprtsock.c +++ b/net/sunrpc/xprtsock.c @@ -473,7 +473,16 @@ static int xs_nospace(struct rpc_task *task) 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; } -- 2.7.4