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>

2 files changed, 41 insertions, 60 deletions

diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
index d06d2a61e31b..f1d3ccb2980e 100644
--- a/fs/xfs/xfs_dquot.c
+++ b/fs/xfs/xfs_dquot.c
@@ -39,20 +39,19 @@
 #include "xfs_qm.h"
 #include "xfs_trace.h"
 
-
 /*
-   LOCK ORDER
-
-   inode lock               (ilock)
-   dquot hash-chain lock    (hashlock)
-   xqm dquot freelist lock  (freelistlock
-   mount's dquot list lock  (mplistlock)
-   user dquot lock - lock ordering among dquots is based on the uid or gid
-   group dquot lock - similar to udquots. Between the two dquots, the udquot
-                      has to be locked first.
-   pin lock - the dquot lock must be held to take this lock.
-   flush lock - ditto.
-*/
+ * Lock order:
+ *
+ * ip->i_lock
+ *   qh->qh_lock
+ *     qi->qi_dqlist_lock
+ *       dquot->q_qlock (xfs_dqlock() and friends)
+ *         dquot->q_flush (xfs_dqflock() and friends)
+ *         xfs_Gqm->qm_dqfrlist_lock
+ *
+ * If two dquots need to be locked the order is user before group/project,
+ * otherwise by the lowest id first, see xfs_dqlock2.
+ */
 
 #ifdef DEBUG
 xfs_buftarg_t *xfs_dqerror_target;
@@ -984,69 +983,49 @@ restart:
  */
 void
 xfs_qm_dqput(
-        xfs_dquot_t     *dqp)
+        struct xfs_dquot        *dqp)
 {
-        xfs_dquot_t     *gdqp;
+        struct xfs_dquot        *gdqp;
 
         ASSERT(dqp->q_nrefs > 0);
         ASSERT(XFS_DQ_IS_LOCKED(dqp));
 
         trace_xfs_dqput(dqp);
 
-        if (dqp->q_nrefs != 1) {
-                dqp->q_nrefs--;
+recurse:
+        if (--dqp->q_nrefs > 0) {
                 xfs_dqunlock(dqp);
                 return;
         }
 
+        trace_xfs_dqput_free(dqp);
+
+        mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
+        if (list_empty(&dqp->q_freelist)) {
+                list_add_tail(&dqp->q_freelist, &xfs_Gqm->qm_dqfrlist);
+                xfs_Gqm->qm_dqfrlist_cnt++;
+        }
+        mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
+
         /*
-         * drop the dqlock and acquire the freelist and dqlock
-         * in the right order; but try to get it out-of-order first
+         * If we just added a udquot to the freelist, then we want to release
+         * the gdquot reference that it (probably) has. Otherwise it'll keep
+         * the gdquot from getting reclaimed.
          */
-        if (!mutex_trylock(&xfs_Gqm->qm_dqfrlist_lock)) {
-                trace_xfs_dqput_wait(dqp);
-                xfs_dqunlock(dqp);
-                mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
-                xfs_dqlock(dqp);
+        gdqp = dqp->q_gdquot;
+        if (gdqp) {
+                xfs_dqlock(gdqp);
+                dqp->q_gdquot = NULL;
         }
+        xfs_dqunlock(dqp);
 
-        while (1) {
-                gdqp = NULL;
-
-                /* We can't depend on nrefs being == 1 here */
-                if (--dqp->q_nrefs == 0) {
-                        trace_xfs_dqput_free(dqp);
-
-                        if (list_empty(&dqp->q_freelist)) {
-                                list_add_tail(&dqp->q_freelist, &xfs_Gqm->qm_dqfrlist);
-                                xfs_Gqm->qm_dqfrlist_cnt++;
-                        }
-
-                        /*
-                         * If we just added a udquot to the freelist, then
-                         * we want to release the gdquot reference that
-                         * it (probably) has. Otherwise it'll keep the
-                         * gdquot from getting reclaimed.
-                         */
-                        if ((gdqp = dqp->q_gdquot)) {
-                                /*
-                                 * Avoid a recursive dqput call
-                                 */
-                                xfs_dqlock(gdqp);
-                                dqp->q_gdquot = NULL;
-                        }
-                }
-                xfs_dqunlock(dqp);
-
-                /*
-                 * If we had a group quota inside the user quota as a hint,
-                 * release it now.
-                 */
-                if (! gdqp)
-                        break;
+        /*
+         * If we had a group quota hint, release it now.
+         */
+        if (gdqp) {
                 dqp = gdqp;
+                goto recurse;
         }
-        mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
 }
 
 /*
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.