Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966003AbcJZM1V (ORCPT ); Wed, 26 Oct 2016 08:27:21 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:55214 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S941113AbcJZM1G (ORCPT ); Wed, 26 Oct 2016 08:27:06 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Trond Myklebust , David Vrabel , Anna Schumaker Subject: [PATCH 4.8 075/140] sunrpc: fix write space race causing stalls Date: Wed, 26 Oct 2016 14:22:15 +0200 Message-Id: <20161026122223.589502337@linuxfoundation.org> X-Mailer: git-send-email 2.10.1 In-Reply-To: <20161026122220.384323763@linuxfoundation.org> References: <20161026122220.384323763@linuxfoundation.org> User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2200 Lines: 69 4.8-stable review patch. If anyone has any objections, please let me know. ------------------ From: David Vrabel 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(). Suggested-by: Trond Myklebust Signed-off-by: David Vrabel Signed-off-by: Anna Schumaker Signed-off-by: Greg Kroah-Hartman --- net/sunrpc/xprtsock.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) --- a/net/sunrpc/xprtsock.c +++ b/net/sunrpc/xprtsock.c @@ -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; }