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Subject: Re: NFS deadlock between 'sync' and commit after unmount....
From: Trond Myklebust <trond.myklebust@primarydata.com>
In-Reply-To: <20140407135001.56ef9f36@notabene.brown>
Date: Mon, 7 Apr 2014 10:10:27 -0400
Cc: Viro Alexander <viro@zeniv.linux.org.uk>, NFS <linux-nfs@vger.kernel.org>
Message-Id: <F59673DE-8047-4FE0-AFF9-6A3DC37348A5@primarydata.com>
References: <20140407135001.56ef9f36@notabene.brown>
To: Brown Neil <neilb@suse.de>, Jan Kara <jack@suse.cz>
Sender: linux-nfs-owner@vger.kernel.org
List-ID: <linux-nfs.vger.kernel.org>


On Apr 6, 2014, at 23:50, NeilBrown <neilb@suse.de> wrote:

> 
> Hi,
> I've just hit a deadlock in NFS that seems very strange.
> The kernel is 3.14-rc8 which some local changes which shouldn't affect the
> deadlocking code.
> 
> Shortly after umounting the NFS filesystem with "umount -f" (though I don't
> think the -f is important), I ran "sync".
> 
> The sync is now stuck in
> 
> [<ffffffff81197fc1>] sync_inodes_sb+0xa1/0x1c0
> [<ffffffff8119cd99>] sync_inodes_one_sb+0x19/0x20
> [<ffffffff81173372>] iterate_supers+0xb2/0x110
> [<ffffffff8119cfd0>] sys_sync+0x30/0x90
> [<ffffffff81aa4622>] system_call_fastpath+0x16/0x1b
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> 
> while kworker/u16:1 is stuck:
> 
> [<ffffffff815420b3>] call_rwsem_down_write_failed+0x13/0x20
> [<ffffffff81172889>] deactivate_super+0x39/0x60
> [<ffffffff812d56f1>] nfs_sb_deactive+0x21/0x30
> [<ffffffff812d2ef9>] __put_nfs_open_context+0xc9/0x100
> [<ffffffff812d2f3b>] put_nfs_open_context+0xb/0x10
> [<ffffffff812ddd14>] nfs_commitdata_release+0x14/0x30
> [<ffffffff812ddd4a>] nfs_commit_release+0x1a/0x20
> [<ffffffff81a45a05>] rpc_free_task+0x25/0x70
> [<ffffffff81a45fd8>] rpc_do_put_task+0x78/0x80
> [<ffffffff81a45feb>] rpc_put_task+0xb/0x10
> [<ffffffff812de3fe>] nfs_initiate_commit+0xce/0x110
> [<ffffffff812df112>] nfs_commit_list+0x62/0x90
> [<ffffffff812dfd26>] nfs_commit_inode+0xa6/0x170
> [<ffffffff812dfe4d>] nfs_write_inode+0x5d/0xa0
> [<ffffffff81300d69>] nfs4_write_inode+0x9/0x10
> [<ffffffff811978ec>] __writeback_single_inode+0x10c/0x2c0
> [<ffffffff811987ea>] writeback_sb_inodes+0x2ca/0x450
> [<ffffffff81198b2c>] wb_writeback+0xec/0x320
> [<ffffffff81199365>] bdi_writeback_workfn+0x115/0x4c0
> [<ffffffff810a595b>] process_one_work+0x16b/0x430
> [<ffffffff810a6619>] worker_thread+0x119/0x3a0
> [<ffffffff810ac2bd>] kthread+0xcd/0xf0
> [<ffffffff81aa457c>] ret_from_fork+0x7c/0xb0
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> 
> So sync is holding sb->s_umount, queued some bdi work on the filesystem and
> is waiting for it to complete.
> Mean while, that work has (I think) submitted a 'commit' (via ->write_inode)
> and that commit wants to deactivate_super and so needs to get ->s_umount.
> 
> I suspect this could happen even more easily with a lazy unmount.
> 
> It seems that this commit request is that last thing that is keeping
> ->s_active elevated and it deadlocks trying to drop the last s_active.
> 
> I have no idea how to fix it....  help?
> 

The problem seems to be the use of iterate_supers(), which grabs a passive reference, and conflicts with our use of an active reference in the open context.

Jan, any suggestions?

Cheers
  Trond

_________________________________
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@primarydata.com