> 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?
Cheers
Trond
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