xfs: implement lazy removal for the dquot freelist

Do not remove dquots from the freelist when we grab a reference to them in
xfs_qm_dqlookup, but leave them on the freelist util scanning notices that
they have a reference.  This speeds up the lookup fastpath, and greatly
simplifies the lock ordering constraints.  Note that the same scheme is
used by the VFS inode and dentry caches.

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

4 files changed, 30 insertions, 65 deletions

diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
index 3f94f2428a35..35d2b8aad0f9 100644
--- a/fs/xfs/xfs_dquot.c
+++ b/fs/xfs/xfs_dquot.c
@@ -722,58 +722,25 @@ xfs_qm_dqlookup(
                  * dqlock to look at the id field of the dquot, since the
                  * id can't be modified without the hashlock anyway.
                  */
-                if (be32_to_cpu(dqp->q_core.d_id) == id && dqp->q_mount == mp) {
-                        trace_xfs_dqlookup_found(dqp);
+                if (be32_to_cpu(dqp->q_core.d_id) != id || dqp->q_mount != mp)
+                        continue;
 
-                        /*
-                         * All in core dquots must be on the dqlist of mp
-                         */
-                        ASSERT(!list_empty(&dqp->q_mplist));
-
-                        xfs_dqlock(dqp);
-                        if (dqp->q_nrefs == 0) {
-                                ASSERT(!list_empty(&dqp->q_freelist));
-                                if (!mutex_trylock(&xfs_Gqm->qm_dqfrlist_lock)) {
-                                        trace_xfs_dqlookup_want(dqp);
-
-                                        /*
-                                         * We may have raced with dqreclaim_one()
-                                         * (and lost). So, flag that we don't
-                                         * want the dquot to be reclaimed.
-                                         */
-                                        dqp->dq_flags |= XFS_DQ_WANT;
-                                        xfs_dqunlock(dqp);
-                                        mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
-                                        xfs_dqlock(dqp);
-                                        dqp->dq_flags &= ~(XFS_DQ_WANT);
-                                }
-
-                                if (dqp->q_nrefs == 0) {
-                                        /* take it off the freelist */
-                                        trace_xfs_dqlookup_freelist(dqp);
-                                        list_del_init(&dqp->q_freelist);
-                                        xfs_Gqm->qm_dqfrlist_cnt--;
-                                }
-                                XFS_DQHOLD(dqp);
-                                mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-                        } else {
-                                XFS_DQHOLD(dqp);
-                        }
+                trace_xfs_dqlookup_found(dqp);
 
-                        /*
-                         * move the dquot to the front of the hashchain
-                         */
-                        ASSERT(mutex_is_locked(&qh->qh_lock));
-                        list_move(&dqp->q_hashlist, &qh->qh_list);
-                        trace_xfs_dqlookup_done(dqp);
-                        *O_dqpp = dqp;
-                        return 0;
-                }
+                xfs_dqlock(dqp);
+                XFS_DQHOLD(dqp);
+
+                /*
+                 * move the dquot to the front of the hashchain
+                 */
+                list_move(&dqp->q_hashlist, &qh->qh_list);
+                trace_xfs_dqlookup_done(dqp);
+                *O_dqpp = dqp;
+                return 0;
         }
 
         *O_dqpp = NULL;
-        ASSERT(mutex_is_locked(&qh->qh_lock));
-        return (1);
+        return 1;
 }
 
 /*
@@ -1033,8 +1000,10 @@ xfs_qm_dqput(
                 if (--dqp->q_nrefs == 0) {
                         trace_xfs_dqput_free(dqp);
 
-                        list_add_tail(&dqp->q_freelist, &xfs_Gqm->qm_dqfrlist);
-                        xfs_Gqm->qm_dqfrlist_cnt++;
+                        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
diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index be1df6839237..6a0c4f0d9306 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -517,13 +517,12 @@ xfs_qm_dqpurge_int(
          * get them off mplist and hashlist, but leave them on freelist.
          */
         list_for_each_entry_safe(dqp, n, &q->qi_dqlist, q_mplist) {
-                /*
-                 * It's OK to look at the type without taking dqlock here.
-                 * We're holding the mplist lock here, and that's needed for
-                 * a dqreclaim.
-                 */
-                if ((dqp->dq_flags & dqtype) == 0)
+                xfs_dqlock(dqp);
+                if ((dqp->dq_flags & dqtype) == 0) {
+                        xfs_dqunlock(dqp);
                         continue;
+                }
+                xfs_dqunlock(dqp);
 
                 if (!mutex_trylock(&dqp->q_hash->qh_lock)) {
                         nrecl = q->qi_dqreclaims;
@@ -1692,14 +1691,15 @@ again:
                 xfs_dqlock(dqp);
 
                 /*
-                 * We are racing with dqlookup here. Naturally we don't
-                 * want to reclaim a dquot that lookup wants. We release the
-                 * freelist lock and start over, so that lookup will grab
-                 * both the dquot and the freelistlock.
+                 * This dquot has already been grabbed by dqlookup.
+                 * Remove it from the freelist and try again.
                  */
-                if (dqp->dq_flags & XFS_DQ_WANT) {
+                if (dqp->q_nrefs) {
                         trace_xfs_dqreclaim_want(dqp);
                         XQM_STATS_INC(xqmstats.xs_qm_dqwants);
+
+                        list_del_init(&dqp->q_freelist);
+                        xfs_Gqm->qm_dqfrlist_cnt--;
                         restarts++;
                         startagain = 1;
                         goto dqunlock;
diff --git a/fs/xfs/xfs_quota.h b/fs/xfs/xfs_quota.h
index cbafdee8b98a..487653ddbef0 100644
--- a/fs/xfs/xfs_quota.h
+++ b/fs/xfs/xfs_quota.h
@@ -87,7 +87,6 @@ typedef struct xfs_dqblk {
 #define XFS_DQ_PROJ             0x0002          /* project quota */
 #define XFS_DQ_GROUP            0x0004          /* a group quota */
 #define XFS_DQ_DIRTY            0x0008          /* dquot is dirty */
-#define XFS_DQ_WANT             0x0010          /* for lookup/reclaim race */
 
 #define XFS_DQ_ALLTYPES         (XFS_DQ_USER|XFS_DQ_PROJ|XFS_DQ_GROUP)
 
@@ -95,8 +94,7 @@ typedef struct xfs_dqblk {
         { XFS_DQ_USER,          "USER" }, \
         { XFS_DQ_PROJ,          "PROJ" }, \
         { XFS_DQ_GROUP,         "GROUP" }, \
-        { XFS_DQ_DIRTY,         "DIRTY" }, \
-        { XFS_DQ_WANT,          "WANT" }
+        { XFS_DQ_DIRTY,         "DIRTY" }
 
 /*
  * In the worst case, when both user and group quotas are on,
diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
index 494035798873..a9d5b1e06efe 100644
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -743,8 +743,6 @@ DEFINE_DQUOT_EVENT(xfs_dqtobp_read);
 DEFINE_DQUOT_EVENT(xfs_dqread);
 DEFINE_DQUOT_EVENT(xfs_dqread_fail);
 DEFINE_DQUOT_EVENT(xfs_dqlookup_found);
-DEFINE_DQUOT_EVENT(xfs_dqlookup_want);
-DEFINE_DQUOT_EVENT(xfs_dqlookup_freelist);
 DEFINE_DQUOT_EVENT(xfs_dqlookup_done);
 DEFINE_DQUOT_EVENT(xfs_dqget_hit);
 DEFINE_DQUOT_EVENT(xfs_dqget_miss);