[PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

[Chris Mason <clm@fb.com>]

Linus, this is the plugging problem I mentioned in my btrfs pull.  It
impacts only MD raid10 and btrfs raid5/6, and I'm not wild about the
patch. But I wanted to at least send in the basic fix for rc1 so this
doesn't cause bigger problems for early testers:

Commit d353d7587 added a plug/finish_plug pair to writeback_sb_inodes,
but writeback_sb_inodes has a horrible secret...it's called with the
wb->list_lock held.

Btrfs raid5/6 and MD raid10 have horrible secrets of their own...they
both do allocations in their unplug callbacks.

None of the options to fix it are very pretty.  We don't want to kick
off workers for all of these unplugs, and the lock doesn't look hot
enough to justify bigger restructuring.

[ 2854.025042] BUG: sleeping function called from invalid context at mm/page_alloc.c:3189
[ 2854.041366] in_atomic(): 1, irqs_disabled(): 0, pid: 145562, name: kworker/u66:15
[ 2854.056813] INFO: lockdep is turned off.
[ 2854.064870] CPU: 13 PID: 145562 Comm: kworker/u66:15 Not tainted 4.2.0-mason+ #1
[ 2854.080082] Hardware name: ZTSYSTEMS Echo Ridge T4  /A9DRPF-10D, BIOS 1.07 05/10/2012
[ 2854.096211] Workqueue: writeback wb_workfn (flush-btrfs-244)
[ 2854.107821]  ffffffff81a2bbee ffff880ee09a7598 ffffffff813307bb ffff880ee09a7598
[ 2854.123162]  ffff881010d1ca00 ffff880ee09a75c8 ffffffff81086615 0000000000000000
[ 2854.138556]  0000000000000000 0000000000000c75 ffffffff81a2bbee ffff880ee09a75f8
[ 2854.153936] Call Trace:
[ 2854.181101]  [<ffffffff81086722>] __might_sleep+0x52/0x90
[ 2854.192136]  [<ffffffff8116d2b4>] __alloc_pages_nodemask+0x344/0xbe0
[ 2854.229682]  [<ffffffff811b54aa>] alloc_pages_current+0x10a/0x1e0
[ 2854.255508]  [<ffffffffa0663f19>] full_stripe_write+0x59/0xc0 [btrfs]
[ 2854.268600]  [<ffffffffa0663fb9>] __raid56_parity_write+0x39/0x60 [btrfs]
[ 2854.282385]  [<ffffffffa06640fb>] run_plug+0x11b/0x140 [btrfs]
[ 2854.294259]  [<ffffffffa0664143>] btrfs_raid_unplug+0x23/0x70 [btrfs]
[ 2854.307334]  [<ffffffff81307622>] blk_flush_plug_list+0x82/0x1f0
[ 2854.319542]  [<ffffffff813077c4>] blk_finish_plug+0x34/0x50
[ 2854.330878]  [<ffffffff812079c2>] writeback_sb_inodes+0x122/0x580
[ 2854.343256]  [<ffffffff81208016>] wb_writeback+0x136/0x4e0

Signed-off-by: Chris Mason <clm@fb.com>
Reviewed-by: Jens Axboe <axboe@fb.com>
---
 fs/fs-writeback.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index ae0f438..07c9c50 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1539,7 +1539,9 @@ static long writeback_sb_inodes(struct super_block *sb,
 				break;
 		}
 	}
+	spin_unlock(&wb->list_lock);
 	blk_finish_plug(&plug);
+	spin_lock(&wb->list_lock);
 	return wrote;
 }
 
-- 
1.8.1