GFS2: Take glock reference in examine_bucket()

We need to check the glock ref counter in a race free way
in order to ensure that the gfs2_glock_hold() call will
succeed. The easiest way to do that is to simply take the
reference count early in the common code of examine_bucket,
skipping any glocks with zero ref count.

That means that the examiner functions all need to put their
reference on the glock once they've performed their function.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Reported-by: David Teigland <teigland@redhat.com>
Tested-by: David Teigland <teigland@redhat.com>

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 544a809819c3ee5c16ddaa42a8ce0ce26d2194f4..ce7078d5aa9787ac1a8ef9dd3798aef2db73de21 100644 (file)
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1488,7 +1488,7 @@ static void examine_bucket(glock_examiner examiner, const struct gfs2_sbd *sdp,
 
        rcu_read_lock();
        hlist_bl_for_each_entry_rcu(gl, pos, head, gl_list) {
-               if ((gl->gl_sbd == sdp) && atomic_read(&gl->gl_ref))
+               if ((gl->gl_sbd == sdp) && atomic_inc_not_zero(&gl->gl_ref))
                        examiner(gl);
        }
        rcu_read_unlock();
@@ -1508,18 +1508,17 @@ static void glock_hash_walk(glock_examiner examiner, const struct gfs2_sbd *sdp)
  * thaw_glock - thaw out a glock which has an unprocessed reply waiting
  * @gl: The glock to thaw
  *
- * N.B. When we freeze a glock, we leave a ref to the glock outstanding,
- * so this has to result in the ref count being dropped by one.
  */
 
 static void thaw_glock(struct gfs2_glock *gl)
 {
        if (!test_and_clear_bit(GLF_FROZEN, &gl->gl_flags))
-               return;
+               goto out;
        set_bit(GLF_REPLY_PENDING, &gl->gl_flags);
-       gfs2_glock_hold(gl);
-       if (queue_delayed_work(glock_workqueue, &gl->gl_work, 0) == 0)
+       if (queue_delayed_work(glock_workqueue, &gl->gl_work, 0) == 0) {
+out:
                gfs2_glock_put(gl);
+       }
 }
 
 /**
@@ -1536,7 +1535,6 @@ static void clear_glock(struct gfs2_glock *gl)
        if (gl->gl_state != LM_ST_UNLOCKED)
                handle_callback(gl, LM_ST_UNLOCKED, 0, false);
        spin_unlock(&gl->gl_spin);
-       gfs2_glock_hold(gl);
        if (queue_delayed_work(glock_workqueue, &gl->gl_work, 0) == 0)
                gfs2_glock_put(gl);
 }