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

On Sep 16, 2016, at 13:29, David Vrabel [off-list ref] wrote:

On 16/09/16 18:06, Trond Myklebust wrote:

quoted

quoted

On Sep 16, 2016, at 12:41, David Vrabel [off-list ref] wrote:

On 16/09/16 17:01, Trond Myklebust wrote:

quoted

quoted

On Sep 16, 2016, at 08:28, David Vrabel [off-list ref] 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 <redacted>
---
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.

quoted

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.