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>

1 files changed, 7 insertions, 7 deletions

diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index 6449bae1570..de1bbad0c7d 100644
--- a/net/9p/trans_fd.c
+++ b/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: