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>

net/9p/trans_fd.c

@@ -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: