Commit 0462194d authored Sep 17, 2012 by Simon Derr Committed by Eric Van Hensbergen Sep 17, 2012

9P: Fix race in p9_read_work()



Race scenario between p9_read_work() and p9_poll_mux()

Data arrive, Rworksched is set, p9_read_work() is called.

thread A                                thread B

                                        p9_read_work()
                                                .
                                        reads data
                                                .
                                        checks if new data ready. No.
                                                .
                                        gets preempted
                                                .
More data arrive, p9_poll_mux() is called.      .
                                                .
                                                .
p9_poll_mux()                                   .
                                                .
if (!test_and_set_bit(Rworksched,               .
                      &m->wsched)) {            .
  schedule_work(&m->rq);                        .
}                                               .
                                                .
-> does not schedule work because               .
   Rworksched is set                            .
                                                .
                                        clear_bit(Rworksched, &m->wsched);
                                        return;

No work has been scheduled, and yet data are waiting.

Currently p9_read_work() checks if there is data to read,
and if not, it clears Rworksched.

I think it should clear Rworksched first, and then check if there is data to read.

Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

parent e549c133

net/9p/trans_fd.c

+7 −7

Original line number	Diff line number	Diff line
		@@ -316,8 +316,7 @@ static void p9_read_work(struct work_struct *work)
		m->rsize - m->rpos);
		p9_debug(P9_DEBUG_TRANS, "mux %p got %d bytes\n", m, err);
		if (err == -EAGAIN) {
		clear_bit(Rworksched, &m->wsched);
		return;
		goto end_clear;
		}

		if (err <= 0)
		@@ -379,19 +378,20 @@ static void p9_read_work(struct work_struct *work)
		m->req = NULL;
		}

		end_clear:
		clear_bit(Rworksched, &m->wsched);

		if (!list_empty(&m->req_list)) {
		if (test_and_clear_bit(Rpending, &m->wsched))
		n = POLLIN;
		else
		n = p9_fd_poll(m->client, NULL);

		if (n & POLLIN) {
		if ((n & POLLIN) && !test_and_set_bit(Rworksched, &m->wsched)) {
		p9_debug(P9_DEBUG_TRANS, "sched read work %p\n", m);
		schedule_work(&m->rq);
		} else
		clear_bit(Rworksched, &m->wsched);
		} else
		clear_bit(Rworksched, &m->wsched);
		}
		}

		return;
		error:

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