| From | Greg Kroah-Hartman <> | Subject | [PATCH 4.8 075/140] sunrpc: fix write space race causing stalls | Date | Wed, 26 Oct 2016 14:22:15 +0200 |
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4.8-stable review patch. If anyone has any objections, please let me know.
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From: David Vrabel <david.vrabel@citrix.com>
commit d48f9ce73c997573e1b512893fa6eddf353a6f69 upstream.
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 resetting the SOCKWQ_ASYNC_NOSPACE bit in xs_nospace() so the second call to sk->sk_write_space() calls xprt_write_space().
Suggested-by: Trond Myklebust <trondmy@primarydata.com> Signed-off-by: David Vrabel <david.vrabel@citrix.com> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--- net/sunrpc/xprtsock.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
--- a/net/sunrpc/xprtsock.c +++ b/net/sunrpc/xprtsock.c @@ -473,7 +473,16 @@ static int xs_nospace(struct rpc_task *t spin_unlock_bh(&xprt->transport_lock); /* Race breaker in case memory is freed before above code is called */ - sk->sk_write_space(sk); + if (ret == -EAGAIN) { + struct socket_wq *wq; + + rcu_read_lock(); + wq = rcu_dereference(sk->sk_wq); + set_bit(SOCKWQ_ASYNC_NOSPACE, &wq->flags); + rcu_read_unlock(); + + sk->sk_write_space(sk); + } return ret; }
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