xfs: flatten the dquot lock ordering

Introduce a new XFS_DQ_FREEING flag that tells lookup and mplist walks
to skip a dquot that is beeing freed, and use this avoid the trylock
on the hash and mplist locks in xfs_qm_dqreclaim_one.  Also simplify
xfs_dqpurge by moving the inodes to a dispose list after marking them
XFS_DQ_FREEING and avoid the locker ordering constraints.

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, 49 insertions, 85 deletions

diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index 6a0c4f0d9306..f418731e90f4 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -398,7 +398,8 @@ again:
         mutex_lock(&q->qi_dqlist_lock);
         list_for_each_entry(dqp, &q->qi_dqlist, q_mplist) {
                 xfs_dqlock(dqp);
-                if (! XFS_DQ_IS_DIRTY(dqp)) {
+                if ((dqp->dq_flags & XFS_DQ_FREEING) ||
+                    !XFS_DQ_IS_DIRTY(dqp)) {
                         xfs_dqunlock(dqp);
                         continue;
                 }
@@ -437,6 +438,7 @@ again:
         /* return ! busy */
         return 0;
 }
+
 /*
  * Release the group dquot pointers the user dquots may be
  * carrying around as a hint. mplist is locked on entry and exit.
@@ -453,6 +455,13 @@ xfs_qm_detach_gdquots(
         ASSERT(mutex_is_locked(&q->qi_dqlist_lock));
         list_for_each_entry(dqp, &q->qi_dqlist, q_mplist) {
                 xfs_dqlock(dqp);
+                if (dqp->dq_flags & XFS_DQ_FREEING) {
+                        xfs_dqunlock(dqp);
+                        mutex_unlock(&q->qi_dqlist_lock);
+                        delay(1);
+                        mutex_lock(&q->qi_dqlist_lock);
+                        goto again;
+                }
                 if ((gdqp = dqp->q_gdquot)) {
                         xfs_dqlock(gdqp);
                         dqp->q_gdquot = NULL;
@@ -489,8 +498,8 @@ xfs_qm_dqpurge_int(
         struct xfs_quotainfo    *q = mp->m_quotainfo;
         struct xfs_dquot        *dqp, *n;
         uint                    dqtype;
-        int                     nrecl;
-        int                     nmisses;
+        int                     nmisses = 0;
+        LIST_HEAD               (dispose_list);
 
         if (!q)
                 return 0;
@@ -509,46 +518,26 @@ xfs_qm_dqpurge_int(
          */
         xfs_qm_detach_gdquots(mp);
 
-      again:
-        nmisses = 0;
-        ASSERT(mutex_is_locked(&q->qi_dqlist_lock));
         /*
-         * Try to get rid of all of the unwanted dquots. The idea is to
-         * get them off mplist and hashlist, but leave them on freelist.
+         * Try to get rid of all of the unwanted dquots.
          */
         list_for_each_entry_safe(dqp, n, &q->qi_dqlist, q_mplist) {
                 xfs_dqlock(dqp);
-                if ((dqp->dq_flags & dqtype) == 0) {
-                        xfs_dqunlock(dqp);
-                        continue;
+                if ((dqp->dq_flags & dqtype) != 0 &&
+                    !(dqp->dq_flags & XFS_DQ_FREEING)) {
+                        if (dqp->q_nrefs == 0) {
+                                dqp->dq_flags |= XFS_DQ_FREEING;
+                                list_move_tail(&dqp->q_mplist, &dispose_list);
+                        } else
+                                nmisses++;
                 }
                 xfs_dqunlock(dqp);
-
-                if (!mutex_trylock(&dqp->q_hash->qh_lock)) {
-                        nrecl = q->qi_dqreclaims;
-                        mutex_unlock(&q->qi_dqlist_lock);
-                        mutex_lock(&dqp->q_hash->qh_lock);
-                        mutex_lock(&q->qi_dqlist_lock);
-
-                        /*
-                         * XXXTheoretically, we can get into a very long
-                         * ping pong game here.
-                         * No one can be adding dquots to the mplist at
-                         * this point, but somebody might be taking things off.
-                         */
-                        if (nrecl != q->qi_dqreclaims) {
-                                mutex_unlock(&dqp->q_hash->qh_lock);
-                                goto again;
-                        }
-                }
-
-                /*
-                 * Take the dquot off the mplist and hashlist. It may remain on
-                 * freelist in INACTIVE state.
-                 */
-                nmisses += xfs_qm_dqpurge(dqp);
         }
         mutex_unlock(&q->qi_dqlist_lock);
+
+        list_for_each_entry_safe(dqp, n, &dispose_list, q_mplist)
+                xfs_qm_dqpurge(dqp);
+
         return nmisses;
 }
 
@@ -1667,25 +1656,16 @@ xfs_qm_init_quotainos(
 
 
 /*
- * Just pop the least recently used dquot off the freelist and
- * recycle it. The returned dquot is locked.
+ * Pop the least recently used dquot off the freelist and recycle it.
  */
-STATIC xfs_dquot_t *
+STATIC struct xfs_dquot *
 xfs_qm_dqreclaim_one(void)
 {
-        xfs_dquot_t     *dqpout;
-        xfs_dquot_t     *dqp;
-        int             restarts;
-        int             startagain;
-
-        restarts = 0;
-        dqpout = NULL;
+        struct xfs_dquot        *dqp;
+        int                     restarts = 0;
 
-        /* lockorder: hashchainlock, freelistlock, mplistlock, dqlock, dqflock */
-again:
-        startagain = 0;
         mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
-
+restart:
         list_for_each_entry(dqp, &xfs_Gqm->qm_dqfrlist, q_freelist) {
                 struct xfs_mount *mp = dqp->q_mount;
                 xfs_dqlock(dqp);
@@ -1701,7 +1681,6 @@ again:
                         list_del_init(&dqp->q_freelist);
                         xfs_Gqm->qm_dqfrlist_cnt--;
                         restarts++;
-                        startagain = 1;
                         goto dqunlock;
                 }
 
@@ -1737,57 +1716,42 @@ again:
                         }
                         goto dqunlock;
                 }
+                xfs_dqfunlock(dqp);
 
                 /*
-                 * We're trying to get the hashlock out of order. This races
-                 * with dqlookup; so, we giveup and goto the next dquot if
-                 * we couldn't get the hashlock. This way, we won't starve
-                 * a dqlookup process that holds the hashlock that is
-                 * waiting for the freelist lock.
+                 * Prevent lookup now that we are going to reclaim the dquot.
+                 * Once XFS_DQ_FREEING is set lookup won't touch the dquot,
+                 * thus we can drop the lock now.
                  */
-                if (!mutex_trylock(&dqp->q_hash->qh_lock)) {
-                        restarts++;
-                        goto dqfunlock;
-                }
+                dqp->dq_flags |= XFS_DQ_FREEING;
+                xfs_dqunlock(dqp);
 
-                /*
-                 * This races with dquot allocation code as well as dqflush_all
-                 * and reclaim code. So, if we failed to grab the mplist lock,
-                 * giveup everything and start over.
-                 */
-                if (!mutex_trylock(&mp->m_quotainfo->qi_dqlist_lock)) {
-                        restarts++;
-                        startagain = 1;
-                        goto qhunlock;
-                }
+                mutex_lock(&dqp->q_hash->qh_lock);
+                list_del_init(&dqp->q_hashlist);
+                dqp->q_hash->qh_version++;
+                mutex_unlock(&dqp->q_hash->qh_lock);
 
-                ASSERT(dqp->q_nrefs == 0);
+                mutex_lock(&mp->m_quotainfo->qi_dqlist_lock);
                 list_del_init(&dqp->q_mplist);
                 mp->m_quotainfo->qi_dquots--;
                 mp->m_quotainfo->qi_dqreclaims++;
-                list_del_init(&dqp->q_hashlist);
-                dqp->q_hash->qh_version++;
+                mutex_unlock(&mp->m_quotainfo->qi_dqlist_lock);
+
+                ASSERT(dqp->q_nrefs == 0);
                 list_del_init(&dqp->q_freelist);
                 xfs_Gqm->qm_dqfrlist_cnt--;
-                dqpout = dqp;
-                mutex_unlock(&mp->m_quotainfo->qi_dqlist_lock);
-qhunlock:
-                mutex_unlock(&dqp->q_hash->qh_lock);
-dqfunlock:
-                xfs_dqfunlock(dqp);
+
+                mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
+                return dqp;
 dqunlock:
                 xfs_dqunlock(dqp);
-                if (dqpout)
-                        break;
                 if (restarts >= XFS_QM_RECLAIM_MAX_RESTARTS)
                         break;
-                if (startagain) {
-                        mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-                        goto again;
-                }
+                goto restart;
         }
+
         mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-        return dqpout;
+        return NULL;
 }
 
 /*