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