GFS2: Fix panic in glock memory shrinker

It is possible for gfs2_shrink_glock_memory() to check a glock for
demotion
that's in the process of being freed by gfs2_glock_put().  In this case,
gfs2_shrink_glock_memory() will acquire a new reference to this glock,
and
then try to free the glock itself when it drops the refernce.  To solve
this, gfs2_shrink_glock_memory() just needs to check if the glock is in
the process of being freed, and if so skip it without ever unlocking the
lru_lock.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Acked-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index fdb796c4f940..827136ee794c 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1314,6 +1314,10 @@ static int gfs2_shrink_glock_memory(int nr, gfp_t gfp_mask)
                 list_del_init(&gl->gl_lru);
                 atomic_dec(&lru_count);
 
+                /* Check if glock is about to be freed */
+                if (atomic_read(&gl->gl_ref) == 0)
+                        continue;
+
                 /* Test for being demotable */
                 if (!test_and_set_bit(GLF_LOCK, &gl->gl_flags)) {
                         gfs2_glock_hold(gl);