linux-kernel - Re: __sb_start_write() && force

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Date:	Thu, 20 Aug 2015 09:26:19 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: __sb_start_write() && force_trylock hack

On Wed, Aug 19, 2015 at 05:00:26PM +0200, Oleg Nesterov wrote:
> On 08/19, Dave Chinner wrote:
> >
> > On Tue, Aug 18, 2015 at 04:49:00PM +0200, Oleg Nesterov wrote:
> > > Jan, Dave, perhaps you can take a look...
> > >
> > > On 08/14, Oleg Nesterov wrote:
> > > >
> > > > Plus another patch which removes the "trylock"
> > > > hack in __sb_start_write().
> > >
> > > I meant the patch we already discussed (attached at the end). And yes,
> > > previously I reported it passed the tests. However, I only ran the same
> > > 'grep -il freeze tests/*/???' tests. When I tried to run all tests, I
> > > got the new reports from lockdep.
> > >
> > > 	[ 2098.281171]  May be due to missing lock nesting notation
> >
> > <groan>
> >
> > > 	[ 2098.288744] 4 locks held by fsstress/50971:
> > > 	[ 2098.293408]  #0:  (sb_writers#13){++++++}, at: [<ffffffff81248d32>] __sb_start_write+0x32/0x40
> > > 	[ 2098.303085]  #1:  (&type->i_mutex_dir_key#4/1){+.+.+.}, at: [<ffffffff8125685f>] filename_create+0x7f/0x170
> > > 	[ 2098.314038]  #2:  (sb_internal#2){++++++}, at: [<ffffffff81248d32>] __sb_start_write+0x32/0x40
> > > 	[ 2098.323711]  #3:  (&type->s_umount_key#54){++++++}, at: [<ffffffffa05a638c>] xfs_flush_inodes+0x1c/0x40 [xfs]
> > > 	[ 2098.334898]
> > > 		       stack backtrace:
> > > 	[ 2098.339762] CPU: 3 PID: 50971 Comm: fsstress Not tainted 4.2.0-rc6+ #27
> > > 	[ 2098.347143] Hardware name: Intel Corporation S2600CP/S2600CP, BIOS RMLSDP.86I.R3.27.D685.1305151734 05/15/2013
> > > 	[ 2098.358303]  0000000000000000 00000000e70ee864 ffff880c05a2b9c8 ffffffff817ee692
> > > 	[ 2098.366603]  0000000000000000 ffffffff826f8030 ffff880c05a2bab8 ffffffff810f45be
> > > 	[ 2098.374900]  0000000000000000 ffff880c05a2bb20 0000000000000000 0000000000000000
> > > 	[ 2098.383197] Call Trace:
> > > 	[ 2098.385930]  [<ffffffff817ee692>] dump_stack+0x45/0x57
> > > 	[ 2098.391667]  [<ffffffff810f45be>] __lock_acquire+0x1e9e/0x2040
> > > 	[ 2098.398177]  [<ffffffff810f310d>] ? __lock_acquire+0x9ed/0x2040
> > > 	[ 2098.404787]  [<ffffffff811d4702>] ? pagevec_lookup_entries+0x22/0x30
> > > 	[ 2098.411879]  [<ffffffff811d5142>] ? truncate_inode_pages_range+0x2b2/0x7e0
> > > 	[ 2098.419551]  [<ffffffff810f542e>] lock_acquire+0xbe/0x150
> > > 	[ 2098.425566]  [<ffffffff81248d32>] ? __sb_start_write+0x32/0x40
> > > 	[ 2098.432079]  [<ffffffff810ede91>] percpu_down_read+0x51/0xa0
> > > 	[ 2098.438396]  [<ffffffff81248d32>] ? __sb_start_write+0x32/0x40
> > > 	[ 2098.444905]  [<ffffffff81248d32>] __sb_start_write+0x32/0x40
> > > 	[ 2098.451244]  [<ffffffffa05a7f84>] xfs_trans_alloc+0x24/0x40 [xfs]
> > > 	[ 2098.458076]  [<ffffffffa059e872>] xfs_inactive_truncate+0x32/0x110 [xfs]
> > > 	[ 2098.465580]  [<ffffffffa059f662>] xfs_inactive+0x102/0x120 [xfs]
> > > 	[ 2098.472308]  [<ffffffffa05a475b>] xfs_fs_evict_inode+0x7b/0xc0 [xfs]
> > > 	[ 2098.479401]  [<ffffffff812642ab>] evict+0xab/0x170
> > > 	[ 2098.484748]  [<ffffffff81264568>] iput+0x1a8/0x230
> > > 	[ 2098.490100]  [<ffffffff8127701c>] sync_inodes_sb+0x14c/0x1d0
> > > 	[ 2098.496439]  [<ffffffffa05a6398>] xfs_flush_inodes+0x28/0x40 [xfs]
> > > 	[ 2098.503361]  [<ffffffffa059e213>] xfs_create+0x5c3/0x6d0 [xfs]
> >
> > Another false positive.
> 
> Confused...
> 
> Dave, I removed your explanation which I can't understand anyway, let
> me remind that I know absolutely nothing about filesystems.
> 
> > But there is no deadlock here
> 
> Yes, we hold SB_FREEZE_WRITE lock, so recursive SB_FREEZE_FS is safe.
> 
> But, this means that the comment in __sb_start_write() is still correct,
> "XFS for example gets freeze protection on internal level twice" is true,
> and we can not remove this force_trylock hack.

You've hit a very rare corner case of a rare corner case. Please
forgive me if I forget such paths - stuff that just works because it
was required to be deadlock free is easy to forget because, well, it
doesn't deadlock.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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