[RFC PATCH v2 3/3] xfs: fix xfsaild hang due to lost wake ups

[Brian Foster]

Running xfstests 273 in a loop reproduces an XFS lockup due to
xfsaild entering idle mode indefinitely. The following
high-level sequence of events leads to the hang:

- xfsaild is running with a cached target lsn
- xfs_ail_push() is invoked, updates ailp->xa_target_lsn and
  invokes wake_up_process(). wake_up_process() returns 0
  because xfsaild is already running.
- xfsaild enters idle mode having met its current target.

Once in the described state, xfs_ail_push() is invoked many
more times with the already set threshold_lsn, but these calls
do not lead to wake_up_process() calls because no further
invocations result in moving the threshold_lsn forward. Add a
flag to xfs_ail to capture whether an issued wake actually
succeeds. If not, continue issuing wakes until we know one has
been successful for the current target.

Signed-off-by: Brian Foster <bfoster at redhat.com>
---
 fs/xfs/xfs_trans_ail.c  |    4 ++--
 fs/xfs/xfs_trans_priv.h |    1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 8bc8aa2..e886785 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -583,7 +583,7 @@ xfs_ail_push(
 
 	lip = xfs_ail_min(ailp);
 	if (!lip || XFS_FORCED_SHUTDOWN(ailp->xa_mount) ||
-	    XFS_LSN_CMP(threshold_lsn, ailp->xa_target) <= 0)
+	    ((XFS_LSN_CMP(threshold_lsn, ailp->xa_target) <= 0) && !ailp->xa_pending_wake))
 		return;
 
 	/*
@@ -594,7 +594,7 @@ xfs_ail_push(
 	xfs_trans_ail_copy_lsn(ailp, &ailp->xa_target, &threshold_lsn);
 	smp_wmb();
 
-	wake_up_process(ailp->xa_task);
+	ailp->xa_pending_wake = !wake_up_process(ailp->xa_task);
 }
 
 /*
diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
index 8ab2ced..62bb4a9 100644
--- a/fs/xfs/xfs_trans_priv.h
+++ b/fs/xfs/xfs_trans_priv.h
@@ -71,6 +71,7 @@ struct xfs_ail {
 	spinlock_t		xa_lock;
 	xfs_lsn_t		xa_last_pushed_lsn;
 	int			xa_log_flush;
+	int			xa_pending_wake;
 };
 
 /*
-- 
1.7.7.6