[PATCH] xfs: re-enable xfsaild idle mode and fix associated races

[Brian Foster <bfoster@redhat.com>]

xfsaild idle mode logic currently leads to a couple hangs:

1.) If xfsaild is rescheduled in during an incremental scan
    (i.e., tout != 0) and the target has been updated since
    the previous run, we can hit the new target and go into
    idle mode with a still populated ail.
2.) A wake up is only issued when the target is pushed forward.
    The wake up can race with xfsaild if it is currently in the
    process of entering idle mode, causing future wake up
    events to be lost.

Both hangs are reproducible by running xfstests 273 in a loop.
Modify xfsaild to enter idle mode only when the ail is empty
and the push target has not been moved forward since the last
push.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---

This is a lightly tested version against the xfs tree. I'll be more
heavily testing a version based on my upstream reproducer tree over the
next few days followed by similar testing on this patch if all goes
well. Sending in advance for review.

 fs/xfs/xfs_trans_ail.c  |   29 ++++++++++++++++++++++++++---
 fs/xfs/xfs_trans_priv.h |    1 +
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 9c51448..0819cd3 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -383,6 +383,12 @@ xfsaild_push(
 	}
 
 	spin_lock(&ailp->xa_lock);
+
+	/* barrier matches the xa_target update in xfs_ail_push() */
+	smp_rmb();
+	target = ailp->xa_target;
+	ailp->xa_target_prev = target;
+
 	lip = xfs_trans_ail_cursor_first(ailp, &cur, ailp->xa_last_pushed_lsn);
 	if (!lip) {
 		/*
@@ -397,7 +403,6 @@ xfsaild_push(
 	XFS_STATS_INC(xs_push_ail);
 
 	lsn = lip->li_lsn;
-	target = ailp->xa_target;
 	while ((XFS_LSN_CMP(lip->li_lsn, target) <= 0)) {
 		int	lock_result;
 
@@ -527,8 +532,26 @@ xfsaild(
 			__set_current_state(TASK_KILLABLE);
 		else
 			__set_current_state(TASK_INTERRUPTIBLE);
-		schedule_timeout(tout ?
-				 msecs_to_jiffies(tout) : MAX_SCHEDULE_TIMEOUT);
+
+		spin_lock(&ailp->xa_lock);
+
+		/* barrier matches the xa_target update in xfs_ail_push() */
+		smp_rmb();
+		if (!xfs_ail_min(ailp) && (ailp->xa_target == ailp->xa_target_prev)) {
+			/* the ail is empty and no change to the push target - idle */
+			spin_unlock(&ailp->xa_lock);
+			schedule();
+			tout = 0;
+			continue;
+		}
+		spin_unlock(&ailp->xa_lock);
+
+		if (tout) {
+			/* more work to do soon */
+			schedule_timeout(msecs_to_jiffies(tout));
+		}
+
+		__set_current_state(TASK_RUNNING);
 
 		try_to_freeze();
 
diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
index fb62377..53b7c9b 100644
--- a/fs/xfs/xfs_trans_priv.h
+++ b/fs/xfs/xfs_trans_priv.h
@@ -67,6 +67,7 @@ struct xfs_ail {
 	struct task_struct	*xa_task;
 	struct list_head	xa_ail;
 	xfs_lsn_t		xa_target;
+	xfs_lsn_t		xa_target_prev;
 	struct list_head	xa_cursors;
 	spinlock_t		xa_lock;
 	xfs_lsn_t		xa_last_pushed_lsn;
-- 
1.7.7.6

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