netdev - Re: [PATCHv1] sunrpc: fix write space race causing stalls

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Message-ID: <4BCBEAB3-1AD4-455E-9C0B-CCD3172C2C17@primarydata.com>
Date:   Fri, 16 Sep 2016 17:06:39 +0000
From:   Trond Myklebust <trondmy@...marydata.com>
To:     David Vrabel <david.vrabel@...rix.com>
CC:     List Linux NFS Mailing <linux-nfs@...r.kernel.org>,
        Schumaker Anna <anna.schumaker@...app.com>,
        List Linux Network Devel Mailing <netdev@...r.kernel.org>
Subject: Re: [PATCHv1] sunrpc: fix write space race causing stalls


> On Sep 16, 2016, at 12:41, David Vrabel <david.vrabel@...rix.com> wrote:
> 
> On 16/09/16 17:01, Trond Myklebust wrote:
>> 
>>> On Sep 16, 2016, at 08:28, David Vrabel <david.vrabel@...rix.com> 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 <david.vrabel@...rix.com>
>>> ---
>>> 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.

Sorry. Copy+paste error. I meant SOCKWQ_ASYNC_NOSPACE.

> 
> 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.

Why not?