linux-kernel - Re: INFO: task hung in sync

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Date:   Thu, 8 Feb 2018 17:20:16 +0100
From:   Jan Kara <jack@...e.cz>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>,
        syzbot <syzbot+283c3c447181741aea28@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, aryabinin@...tuozzo.com,
        jlayton@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mgorman@...hsingularity.net, mingo@...nel.org,
        rgoldwyn@...e.com, syzkaller-bugs@...glegroups.com,
        linux-fsdevel@...r.kernel.org
Subject: Re: INFO: task hung in sync_blockdev

On Thu 08-02-18 06:49:18, Andi Kleen wrote:
> > > It seems multiple processes deadlocked on the bd_mutex. 
> > > Unfortunately there's no backtrace for the lock acquisitions,
> > > so it's hard to see the exact sequence.
> > 
> > Well, all in the report points to a situation where some IO was submitted
> > to the block device and never completed (more exactly it took longer than
> > those 120s to complete that IO). It would need more digging into the
> 
> Are you sure? I didn't think outstanding IO would take bd_mutex.

The stack trace is:

 schedule+0xf5/0x430 kernel/sched/core.c:3480
 io_schedule+0x1c/0x70 kernel/sched/core.c:5096
 wait_on_page_bit_common+0x4b3/0x770 mm/filemap.c:1099
 wait_on_page_bit mm/filemap.c:1132 [inline]
 wait_on_page_writeback include/linux/pagemap.h:546 [inline]
 __filemap_fdatawait_range+0x282/0x430 mm/filemap.c:533
 filemap_fdatawait_range mm/filemap.c:558 [inline]
 filemap_fdatawait include/linux/fs.h:2590 [inline]
 filemap_write_and_wait+0x7a/0xd0 mm/filemap.c:624
 __sync_blockdev fs/block_dev.c:448 [inline]
 sync_blockdev.part.29+0x50/0x70 fs/block_dev.c:457
 sync_blockdev fs/block_dev.c:444 [inline]
 __blkdev_put+0x18b/0x7f0 fs/block_dev.c:1763
 blkdev_put+0x85/0x4f0 fs/block_dev.c:1835
 blkdev_close+0x8b/0xb0 fs/block_dev.c:1842
 __fput+0x327/0x7e0 fs/file_table.c:209
 ____fput+0x15/0x20 fs/file_table.c:243


So we are waiting for PageWriteback on some page. And bd_mutex is grabbed
by this process in __blkdev_put() before calling sync_blockdev().

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR