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9P: Fix race in p9_read_work()

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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>
This commit is contained in:
Simon Derr 2012-09-17 15:16:28 +02:00 committed by Eric Van Hensbergen
parent e549c13371
commit 0462194d35
1 changed files with 7 additions and 7 deletions
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14
net/9p/trans_fd.c
View File

@ -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: