xfs: nest qm_dqfrlist_lock inside the dquot qlock

Allow xfs_qm_dqput to work without trylock loops by nesting the freelist lock
inside the dquot qlock.  In turn that requires trylocks in the reclaim path
instead, but given it's a classic tradeoff between fast and slow path, and
we follow the model of the inode and dentry caches.

Document our new lock order now that it has settled.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index f418731e90f4..22360bb26af9 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -1668,7 +1668,9 @@ xfs_qm_dqreclaim_one(void)
 restart:
         list_for_each_entry(dqp, &xfs_Gqm->qm_dqfrlist, q_freelist) {
                 struct xfs_mount *mp = dqp->q_mount;
-                xfs_dqlock(dqp);
+
+                if (!xfs_dqlock_nowait(dqp))
+                        continue;
 
                 /*
                  * This dquot has already been grabbed by dqlookup.