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9P: Fix race between p9_write_work() and p9_fd_request()

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Race scenario:

thread A			thread B

p9_write_work()                p9_fd_request()

if (list_empty
  (&m->unsent_req_list))
  ...

                               spin_lock(&client->lock);
                               req->status = REQ_STATUS_UNSENT;
                               list_add_tail(..., &m->unsent_req_list);
                               spin_unlock(&client->lock);
                               ....
                               if (n & POLLOUT &&
                               !test_and_set_bit(Wworksched, &m->wsched)
                               schedule_work(&m->wq);
                               --> not done because Wworksched is set

  clear_bit(Wworksched, &m->wsched);
  return;

--> nobody will take care of sending the new request.

This is not very likely to happen though, because p9_write_work()
being called with an empty unsent_req_list is not frequent.
But this also means that taking the lock earlier will not be costly.

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 15:16:31 +02:00 committed by Eric Van Hensbergen
parent 584a8c13d5
commit 759f42987f
1 changed files with 2 additions and 1 deletions
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3
net/9p/trans_fd.c
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@ -453,12 +453,13 @@ static void p9_write_work(struct work_struct *work)
}
if (!m->wsize) {
spin_lock(&m->client->lock);
if (list_empty(&m->unsent_req_list)) {
clear_bit(Wworksched, &m->wsched);
spin_unlock(&m->client->lock);
return;
}
spin_lock(&m->client->lock);
req = list_entry(m->unsent_req_list.next, struct p9_req_t,
req_list);
req->status = REQ_STATUS_SENT;