xfs: fix dquot shaker deadlock

Commit 368e1361 ("xfs: remove duplicate code from dquot reclaim") fails
to unlock the dquot freelist when the number of loop restarts is
exceeded in xfs_qm_dqreclaim_one(). This causes hangs in memory
reclaim.

Rework the loop control logic into an unwind stack that all the
different cases jump into. This means there is only one set of code
that processes the loop exit criteria, and simplifies the unlocking
of all the items from different points in the loop. It also fixes a
double increment of the restart counter from the qi_dqlist_lock
case.
Reported-by: Malcolm Scott <lkml@malc.org.uk>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Alex Elder <aelder@sgi.com>

fs/xfs/quota/xfs_qm.c

@@ -1863,12 +1863,14 @@ xfs_qm_dqreclaim_one(void)
 	xfs_dquot_t	*dqpout;
 	xfs_dquot_t	*dqp;
 	int		restarts;
+	int		startagain;
 
 	restarts = 0;
 	dqpout = NULL;
 
 	/* lockorder: hashchainlock, freelistlock, mplistlock, dqlock, dqflock */
-startagain:
+again:
+	startagain = 0;
 	mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
 
 	list_for_each_entry(dqp, &xfs_Gqm->qm_dqfrlist, q_freelist) {
@@ -1885,13 +1887,10 @@ xfs_qm_dqreclaim_one(void)
 			ASSERT(! (dqp->dq_flags & XFS_DQ_INACTIVE));
 
 			trace_xfs_dqreclaim_want(dqp);
-
-			xfs_dqunlock(dqp);
-			mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-			if (++restarts >= XFS_QM_RECLAIM_MAX_RESTARTS)
-				return NULL;
 			XQM_STATS_INC(xqmstats.xs_qm_dqwants);
-			goto startagain;
+			restarts++;
+			startagain = 1;
+			goto dqunlock;
 		}
 
 		/*
@@ -1906,23 +1905,20 @@ xfs_qm_dqreclaim_one(void)
 			ASSERT(list_empty(&dqp->q_mplist));
 			list_del_init(&dqp->q_freelist);
 			xfs_Gqm->qm_dqfrlist_cnt--;
-			xfs_dqunlock(dqp);
 			dqpout = dqp;
 			XQM_STATS_INC(xqmstats.xs_qm_dqinact_reclaims);
-			break;
+			goto dqunlock;
 		}
 
 		ASSERT(dqp->q_hash);
 		ASSERT(!list_empty(&dqp->q_mplist));
 
 		/*
-		 * Try to grab the flush lock. If this dquot is in the process of
-		 * getting flushed to disk, we don't want to reclaim it.
+		 * Try to grab the flush lock. If this dquot is in the process
+		 * of getting flushed to disk, we don't want to reclaim it.
 		 */
-		if (!xfs_dqflock_nowait(dqp)) {
-			xfs_dqunlock(dqp);
-			continue;
-		}
+		if (!xfs_dqflock_nowait(dqp))
+			goto dqunlock;
 
 		/*
 		 * We have the flush lock so we know that this is not in the
@@ -1944,8 +1940,7 @@ xfs_qm_dqreclaim_one(void)
 				xfs_fs_cmn_err(CE_WARN, mp,
 			"xfs_qm_dqreclaim: dquot %p flush failed", dqp);
 			}
-			xfs_dqunlock(dqp); /* dqflush unlocks dqflock */
-			continue;
+			goto dqunlock;
 		}
 
 		/*
@@ -1967,13 +1962,8 @@ xfs_qm_dqreclaim_one(void)
 		 */
 		if (!mutex_trylock(&mp->m_quotainfo->qi_dqlist_lock)) {
 			restarts++;
-			mutex_unlock(&dqp->q_hash->qh_lock);
-			xfs_dqfunlock(dqp);
-			xfs_dqunlock(dqp);
-			mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
-			if (restarts++ >= XFS_QM_RECLAIM_MAX_RESTARTS)
-				return NULL;
-			goto startagain;
+			startagain = 1;
+			goto qhunlock;
 		}
 
 		ASSERT(dqp->q_nrefs == 0);
@@ -1986,14 +1976,20 @@ xfs_qm_dqreclaim_one(void)
 		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);
+dqunlock:
 		xfs_dqunlock(dqp);
 		if (dqpout)
 			break;
 		if (restarts >= XFS_QM_RECLAIM_MAX_RESTARTS)
-			return NULL;
+			break;
+		if (startagain) {
+			mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
+			goto again;
+		}
 	}
 	mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
 	return dqpout;