> On Sep 16, 2016, at 13:29, David Vrabel <david.vra...@citrix.com> wrote:
>
> On 16/09/16 18:06, Trond Myklebust wrote:
>>
>>> On Sep 16, 2016, at 12:41, David Vrabel <david.vra...@citrix.com> wrote:
>>>
>>> On 16/09/16 17:01, Trond Myklebust wrote:
>>>>
>>>>> On Sep 16, 2016, at 08:28, David Vrabel <david.vra...@citrix.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.vra...@citrix.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?
>
> I prefer my solution because:
>
> a) It obviously fixes the race (games with bits are less understandable).
>
> b) It requires fewer atomic ops.
>
> c) It doesn't require me to understand what the behaviour of the
> socket-internal SOCKWQ_ASYNC_NOSPACE bit is or should be.
>
> d) I'm not sure I understand the objection to the additional
> have_write_space method -- it has simple, clear behaviour.
>
I don’t see the point of adding 24 lines of code over 3 different files if the
problem can be solved with 1 line of code.
