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>

4 files changed, 103 insertions, 150 deletions

diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
index 35d2b8aad0f9..d06d2a61e31b 100644
--- a/fs/xfs/xfs_dquot.c
+++ b/fs/xfs/xfs_dquot.c
@@ -728,6 +728,12 @@ xfs_qm_dqlookup(
                 trace_xfs_dqlookup_found(dqp);
 
                 xfs_dqlock(dqp);
+                if (dqp->dq_flags & XFS_DQ_FREEING) {
+                        *O_dqpp = NULL;
+                        xfs_dqunlock(dqp);
+                        return -1;
+                }
+
                 XFS_DQHOLD(dqp);
 
                 /*
@@ -781,11 +787,7 @@ xfs_qm_dqget(
                         return (EIO);
                 }
         }
-#endif
 
- again:
-
-#ifdef DEBUG
         ASSERT(type == XFS_DQ_USER ||
                type == XFS_DQ_PROJ ||
                type == XFS_DQ_GROUP);
@@ -797,13 +799,21 @@ xfs_qm_dqget(
                         ASSERT(ip->i_gdquot == NULL);
         }
 #endif
+
+restart:
         mutex_lock(&h->qh_lock);
 
         /*
          * Look in the cache (hashtable).
          * The chain is kept locked during lookup.
          */
-        if (xfs_qm_dqlookup(mp, id, h, O_dqpp) == 0) {
+        switch (xfs_qm_dqlookup(mp, id, h, O_dqpp)) {
+        case -1:
+                XQM_STATS_INC(xqmstats.xs_qm_dquot_dups);
+                mutex_unlock(&h->qh_lock);
+                delay(1);
+                goto restart;
+        case 0:
                 XQM_STATS_INC(xqmstats.xs_qm_dqcachehits);
                 /*
                  * The dquot was found, moved to the front of the chain,
@@ -814,9 +824,11 @@ xfs_qm_dqget(
                 ASSERT(XFS_DQ_IS_LOCKED(*O_dqpp));
                 mutex_unlock(&h->qh_lock);
                 trace_xfs_dqget_hit(*O_dqpp);
-                return (0);     /* success */
+                return 0;       /* success */
+        default:
+                XQM_STATS_INC(xqmstats.xs_qm_dqcachemisses);
+                break;
         }
-        XQM_STATS_INC(xqmstats.xs_qm_dqcachemisses);
 
         /*
          * Dquot cache miss. We don't want to keep the inode lock across
@@ -913,16 +925,21 @@ xfs_qm_dqget(
                  * lock order between the two dquots here since dqp isn't
                  * on any findable lists yet.
                  */
-                if (xfs_qm_dqlookup(mp, id, h, &tmpdqp) == 0) {
+                switch (xfs_qm_dqlookup(mp, id, h, &tmpdqp)) {
+                case 0:
+                case -1:
                         /*
-                         * Duplicate found. Just throw away the new dquot
-                         * and start over.
+                         * Duplicate found, either in cache or on its way out.
+                         * Just throw away the new dquot and start over.
                          */
-                        xfs_qm_dqput(tmpdqp);
+                        if (tmpdqp)
+                                xfs_qm_dqput(tmpdqp);
                         mutex_unlock(&h->qh_lock);
                         xfs_qm_dqdestroy(dqp);
                         XQM_STATS_INC(xqmstats.xs_qm_dquot_dups);
-                        goto again;
+                        goto restart;
+                default:
+                        break;
                 }
         }
 
@@ -1250,51 +1267,18 @@ xfs_dqlock2(
         }
 }
 
-
 /*
- * Take a dquot out of the mount's dqlist as well as the hashlist.
- * This is called via unmount as well as quotaoff, and the purge
- * will always succeed unless there are soft (temp) references
- * outstanding.
- *
- * This returns 0 if it was purged, 1 if it wasn't. It's not an error code
- * that we're returning! XXXsup - not cool.
+ * Take a dquot out of the mount's dqlist as well as the hashlist.  This is
+ * called via unmount as well as quotaoff, and the purge will always succeed.
  */
-/* ARGSUSED */
-int
+void
 xfs_qm_dqpurge(
-        xfs_dquot_t     *dqp)
+        struct xfs_dquot        *dqp)
 {
-        xfs_dqhash_t    *qh = dqp->q_hash;
-        xfs_mount_t     *mp = dqp->q_mount;
-
-        ASSERT(mutex_is_locked(&mp->m_quotainfo->qi_dqlist_lock));
-        ASSERT(mutex_is_locked(&dqp->q_hash->qh_lock));
-
-        /*
-         * XXX(hch): horrible locking order, will get cleaned up ASAP.
-         */
-        if (!mutex_trylock(&xfs_Gqm->qm_dqfrlist_lock)) {
-                mutex_unlock(&dqp->q_hash->qh_lock);
-                return 1;
-        }
+        struct xfs_mount        *mp = dqp->q_mount;
+        struct xfs_dqhash       *qh = dqp->q_hash;
 
         xfs_dqlock(dqp);
-        /*
-         * We really can't afford to purge a dquot that is
-         * referenced, because these are hard refs.
-         * It shouldn't happen in general because we went thru _all_ inodes in
-         * dqrele_all_inodes before calling this and didn't let the mountlock go.
-         * However it is possible that we have dquots with temporary
-         * references that are not attached to an inode. e.g. see xfs_setattr().
-         */
-        if (dqp->q_nrefs != 0) {
-                xfs_dqunlock(dqp);
-                mutex_unlock(&dqp->q_hash->qh_lock);
-                return (1);
-        }
-
-        ASSERT(!list_empty(&dqp->q_freelist));
 
         /*
          * If we're turning off quotas, we have to make sure that, for
@@ -1313,19 +1297,14 @@ xfs_qm_dqpurge(
         }
 
         /*
-         * XXXIf we're turning this type of quotas off, we don't care
+         * If we are turning this type of quotas off, we don't care
          * about the dirty metadata sitting in this dquot. OTOH, if
          * we're unmounting, we do care, so we flush it and wait.
          */
         if (XFS_DQ_IS_DIRTY(dqp)) {
                 int     error;
 
-                /* dqflush unlocks dqflock */
                 /*
-                 * Given that dqpurge is a very rare occurrence, it is OK
-                 * that we're holding the hashlist and mplist locks
-                 * across the disk write. But, ... XXXsup
-                 *
                  * We don't care about getting disk errors here. We need
                  * to purge this dquot anyway, so we go ahead regardless.
                  */
@@ -1335,28 +1314,36 @@ xfs_qm_dqpurge(
                                 __func__, dqp);
                 xfs_dqflock(dqp);
         }
+
         ASSERT(atomic_read(&dqp->q_pincount) == 0);
         ASSERT(XFS_FORCED_SHUTDOWN(mp) ||
                !(dqp->q_logitem.qli_item.li_flags & XFS_LI_IN_AIL));
 
+        xfs_dqfunlock(dqp);
+        xfs_dqunlock(dqp);
+
+        mutex_lock(&qh->qh_lock);
         list_del_init(&dqp->q_hashlist);
         qh->qh_version++;
+        mutex_unlock(&qh->qh_lock);
 
+        mutex_lock(&mp->m_quotainfo->qi_dqlist_lock);
         list_del_init(&dqp->q_mplist);
         mp->m_quotainfo->qi_dqreclaims++;
         mp->m_quotainfo->qi_dquots--;
+        mutex_unlock(&mp->m_quotainfo->qi_dqlist_lock);
 
+        /*
+         * We move dquots to the freelist as soon as their reference count
+         * hits zero, so it really should be on the freelist here.
+         */
+        mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
+        ASSERT(!list_empty(&dqp->q_freelist));
         list_del_init(&dqp->q_freelist);
         xfs_Gqm->qm_dqfrlist_cnt--;
-
-        xfs_dqfunlock(dqp);
-        xfs_dqunlock(dqp);
-
         mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-        mutex_unlock(&qh->qh_lock);
 
         xfs_qm_dqdestroy(dqp);
-        return 0;
 }
 
 /*
diff --git a/fs/xfs/xfs_dquot.h b/fs/xfs/xfs_dquot.h
index 0b5d2ae92028..98488dfe442f 100644
--- a/fs/xfs/xfs_dquot.h
+++ b/fs/xfs/xfs_dquot.h
@@ -133,7 +133,7 @@ static inline void xfs_dqunlock_nonotify(struct xfs_dquot *dqp)
 
 extern void             xfs_qm_dqdestroy(xfs_dquot_t *);
 extern int              xfs_qm_dqflush(xfs_dquot_t *, uint);
-extern int              xfs_qm_dqpurge(xfs_dquot_t *);
+extern void             xfs_qm_dqpurge(xfs_dquot_t *);
 extern void             xfs_qm_dqunpin_wait(xfs_dquot_t *);
 extern void             xfs_qm_adjust_dqtimers(xfs_mount_t *,
                                         xfs_disk_dquot_t *);
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;
 }
 
 /*
diff --git a/fs/xfs/xfs_quota.h b/fs/xfs/xfs_quota.h
index 487653ddbef0..b86c62f5eeba 100644
--- a/fs/xfs/xfs_quota.h
+++ b/fs/xfs/xfs_quota.h
@@ -87,6 +87,7 @@ 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_FREEING          0x0010          /* dquot is beeing torn down */
 
 #define XFS_DQ_ALLTYPES         (XFS_DQ_USER|XFS_DQ_PROJ|XFS_DQ_GROUP)
 
@@ -94,7 +95,8 @@ typedef struct xfs_dqblk {
         { XFS_DQ_USER,          "USER" }, \
         { XFS_DQ_PROJ,          "PROJ" }, \
         { XFS_DQ_GROUP,         "GROUP" }, \
-        { XFS_DQ_DIRTY,         "DIRTY" }
+        { XFS_DQ_DIRTY,         "DIRTY" }, \
+        { XFS_DQ_FREEING,       "FREEING" }
 
 /*
  * In the worst case, when both user and group quotas are on,