[Cluster-devel] GFS2: Fix panic in glock memory shrinker

[Benjamin Marzinski <bmarzins@redhat.com>]

To: cluster-devel@redhat.com
Subject: 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>
---
 fs/gfs2/glock.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: kernel-upstream/fs/gfs2/glock.c
===================================================================
--- kernel-upstream.orig/fs/gfs2/glock.c
+++ kernel-upstream/fs/gfs2/glock.c
@@ -1314,6 +1314,10 @@ static int gfs2_shrink_glock_memory(int 
 		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);