linux-kernel - [PATCH] writeback: fix delayed sync(2)

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Date:	Fri, 20 Sep 2013 16:52:26 +0400
From:	Maxim Patlasov <MPatlasov@...allels.com>
To:	linux-mm@...ck.org
Cc:	tj@...nel.org, akpm@...ux-foundation.org, fengguang.wu@...el.com,
	jack@...e.cz, linux-kernel@...r.kernel.org
Subject: [PATCH] writeback: fix delayed sync(2)

Problem statement: if sync(2) races with bdi_wakeup_thread_delayed
(which is called when the first inode for a bdi is marked dirty), it's
possible that sync will be delayed for long (5 secs if dirty_writeback_interval
is set to default value).

How it works: sync schedules bdi work for immediate processing by calling
mod_delayed_work with 'delay' equal to 0. Bdi work is queued to pool->worklist
and wake_up_worker(pool) is called, but before worker gets the work from
the list, __mark_inode_dirty intervenes calling bdi_wakeup_thread_delayed
who calls mod_delayed_work with 'timeout' equal to dirty_writeback_interval
multiplied by 10. mod_delayed_work dives into try_to_grab_pending who
successfully steals the work from the worklist. Then it's re-queued with that
new delay. Until the timeout is lapsed, sync(2) sits on wait_for_completion in
sync_inodes_sb.

The patch uses queue_delayed_work for __mark_inode_dirty. This should be safe
because even if queue_delayed_work returns false (if the work is already on
a queue), bdi_writeback_workfn will re-schedule itself by looking at
wb->b_dirty.

Signed-off-by: Maxim Patlasov <MPatlasov@...allels.com>
---
 mm/backing-dev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index ce682f7..3fde024 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -294,7 +294,7 @@ void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
 	unsigned long timeout;

 	timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
-	mod_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
+	queue_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
 }

 /*

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