[Cluster-devel] [PATCH] GFS2: Hold gl_hash_table bucket locks in glock_hash_walk

[Andrew Price]

This lockdep splat was being triggered on umount:

[55715.973122] ===============================
[55715.980169] [ INFO: suspicious RCU usage. ]
[55715.981021] 4.3.0-11553-g8d3de01-dirty #15 Tainted: G        W
[55715.982353] -------------------------------
[55715.983301] fs/gfs2/glock.c:1427 suspicious rcu_dereference_protected() usage!

The code it refers to is the rht_for_each_entry_safe usage in
glock_hash_walk. The condition that triggers the warning is
lockdep_rht_bucket_is_held(tbl, hash) which is checked in the
__rcu_dereference_protected macro.

lockdep_rht_bucket_is_held returns false when rht_bucket_lock(tbl, hash)
is not held, which suggests that glock_hash_walk should be holding the
lock for each rhashtable bucket it looks at. Holding those locks clears
up the warning.

Signed-off-by: Andrew Price <anprice at redhat.com>
---
 fs/gfs2/glock.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 32e7471..7384cbb 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1424,11 +1424,13 @@ static void glock_hash_walk(glock_examiner examiner, const struct gfs2_sbd *sdp)
 	rcu_read_lock();
 	tbl = rht_dereference_rcu(gl_hash_table.tbl, &gl_hash_table);
 	for (i = 0; i < tbl->size; i++) {
+		spin_lock(rht_bucket_lock(tbl, i));
 		rht_for_each_entry_safe(gl, pos, next, tbl, i, gl_node) {
 			if ((gl->gl_name.ln_sbd == sdp) &&
 			    lockref_get_not_dead(&gl->gl_lockref))
 				examiner(gl);
 		}
+		spin_unlock(rht_bucket_lock(tbl, i));
 	}
 	rcu_read_unlock();
 	cond_resched();
-- 
2.4.3