Return-Path: Received: from smtp.citrix.com ([66.165.176.89]:1864 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935667AbcIPQmv (ORCPT ); Fri, 16 Sep 2016 12:42:51 -0400 Subject: Re: [PATCHv1] sunrpc: fix write space race causing stalls To: Trond Myklebust References: <1474028902-19838-1-git-send-email-david.vrabel@citrix.com> CC: List Linux NFS Mailing , Schumaker Anna , List Linux Network Devel Mailing From: David Vrabel Message-ID: <57DC20C8.5000306@citrix.com> Date: Fri, 16 Sep 2016 17:41:44 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="windows-1252" Sender: linux-nfs-owner@vger.kernel.org List-ID: On 16/09/16 17:01, Trond Myklebust wrote: > >> On Sep 16, 2016, at 08:28, David Vrabel wrote: >> >> 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 have xprt_wait_for_buffer_space() check for write >> space after putting the task to sleep. >> >> Signed-off-by: David Vrabel >> --- >> include/linux/sunrpc/xprt.h | 1 + >> net/sunrpc/xprt.c | 4 ++++ >> net/sunrpc/xprtsock.c | 21 +++++++++++++++++++-- >> 3 files changed, 24 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h >> index a16070d..621e74b 100644 >> --- a/include/linux/sunrpc/xprt.h >> +++ b/include/linux/sunrpc/xprt.h >> @@ -129,6 +129,7 @@ struct rpc_xprt_ops { >> void (*connect)(struct rpc_xprt *xprt, struct rpc_task *task); >> void * (*buf_alloc)(struct rpc_task *task, size_t size); >> void (*buf_free)(void *buffer); >> + bool (*have_write_space)(struct rpc_xprt *task); >> int (*send_request)(struct rpc_task *task); >> void (*set_retrans_timeout)(struct rpc_task *task); >> void (*timer)(struct rpc_xprt *xprt, struct rpc_task *task); >> diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c >> index ea244b2..d3c1b1e 100644 >> --- a/net/sunrpc/xprt.c >> +++ b/net/sunrpc/xprt.c >> @@ -502,6 +502,10 @@ void xprt_wait_for_buffer_space(struct rpc_task *task, rpc_action action) >> >> task->tk_timeout = RPC_IS_SOFT(task) ? req->rq_timeout : 0; >> rpc_sleep_on(&xprt->pending, task, action); >> + >> + /* Write space notification may race with putting task to sleep. */ >> + if (xprt->ops->have_write_space(xprt)) >> + rpc_wake_up_queued_task(&xprt->pending, task); >> } >> EXPORT_SYMBOL_GPL(xprt_wait_for_buffer_space); >> >> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c >> index bf16883..211de5b 100644 >> --- a/net/sunrpc/xprtsock.c >> +++ b/net/sunrpc/xprtsock.c >> @@ -472,8 +472,6 @@ 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); >> return ret; >> } > > Instead of these callbacks, why not just add a call to > sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk) after queueing the task in > xs_nospace()? Won?t that fix the existing race breaker? I don't see how that would help. If sk->sk_write_space was already called, SOCKWQ_ASYNC_NOSPACE will still be clear and the next call to sk->sk_write_space will still be a nop. Or did you mean SOCKWQ_ASYNC_NOSPACE here? It doesn't seem right to set this bit when we don't know if there's space or not. David