On 12/11/15 20:38, Steven Whitehouse wrote:
Hi,
On 12/11/15 20:11, Bob Peterson wrote:
----- Original Message -----
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 <anpr...@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();
--
Not sure that this is the correct fix. There is nothing changing in the
examiner function that I can see which would require the bucket lock,
and we already have this with an rcu_read_lock, which is correct since
we are only reading the hash lists, not changing them.
Good point, I wasn't sure whether it was safe to assume the examiner
functions weren't changing the buckets and erred on the side of caution.
rht_for_each_entry_safe() does an unconditional
rcu_dereference_protected(p, lockdep_rht_bucket_is_held(tbl, hash))
so we're likely using the wrong rht_for_each_entry* macro. I'm currently
testing a version which uses rht_for_each_entry_rcu() instead. That one uses
rcu_dereference_check(p, lockdep_rht_bucket_is_held(tbl, hash))
which is defined as
#define rcu_dereference_check(p, c) \
__rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
so that looks like the right one to use. I'll send that patch shortly.
Andy
