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From: Miklos Szeredi <miklos@szeredi.hu>
To: CAI Qian <caiqian@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [4.9-rc1+] overlayfs lockdep
Date: Mon, 24 Oct 2016 14:57:17 +0200
Message-ID: <CAJfpegsOtLcV9KCy+UPQr_P2rixAMYBxEjqutpLvh2wEOw+eQw@mail.gmail.com> (raw)
In-Reply-To: <1268904273.1449131.1477064310378.JavaMail.zimbra@redhat.com>

On Fri, Oct 21, 2016 at 5:38 PM, CAI Qian <caiqian@redhat.com> wrote:
>
> ----- Original Message -----
>> From: "Jan Kara" <jack@suse.cz>
>> Sent: Friday, October 7, 2016 3:08:38 AM
>> Subject: Re: local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked)
>>
>>
>> So I believe this may be just a problem in overlayfs lockdep annotation
>> (see below). Added Miklos to CC.
>>
>> > Wait. There is also a lockep happened before the xfs internal error as
>> > well.
>> >
>> > [ 5839.452325] ======================================================
>> > [ 5839.459221] [ INFO: possible circular locking dependency detected ]
>> > [ 5839.466215] 4.8.0-rc8-splice-fixw-proc+ #4 Not tainted
>> > [ 5839.471945] -------------------------------------------------------
>> > [ 5839.478937] trinity-c220/69531 is trying to acquire lock:
>> > [ 5839.484961]  (&p->lock){+.+.+.}, at: [<ffffffff812ac69c>]
>> > seq_read+0x4c/0x3e0
>> > [ 5839.492967]
>> > but task is already holding lock:
>> > [ 5839.499476]  (sb_writers#8){.+.+.+}, at: [<ffffffff81284be1>]
>> > __sb_start_write+0xd1/0xf0
>> > [ 5839.508560]
>> > which lock already depends on the new lock.
>> >
>> > [ 5839.517686]
>> > the existing dependency chain (in reverse order) is:
>> > [ 5839.526036]
>> > -> #3 (sb_writers#8){.+.+.+}:
>> > [ 5839.530751]        [<ffffffff810ff174>] lock_acquire+0xd4/0x240
>> > [ 5839.537368]        [<ffffffff810f8f4a>] percpu_down_read+0x4a/0x90
>> > [ 5839.544275]        [<ffffffff81284be1>] __sb_start_write+0xd1/0xf0
>> > [ 5839.551181]        [<ffffffff812a8544>] mnt_want_write+0x24/0x50
>> > [ 5839.557892]        [<ffffffffa04a398f>] ovl_want_write+0x1f/0x30
>> > [overlay]
>> > [ 5839.565577]        [<ffffffffa04a6036>] ovl_do_remove+0x46/0x480
>> > [overlay]
>> > [ 5839.573259]        [<ffffffffa04a64a3>] ovl_unlink+0x13/0x20 [overlay]
>> > [ 5839.580555]        [<ffffffff812918ea>] vfs_unlink+0xda/0x190
>> > [ 5839.586979]        [<ffffffff81293698>] do_unlinkat+0x268/0x2b0
>> > [ 5839.593599]        [<ffffffff8129419b>] SyS_unlinkat+0x1b/0x30
>> > [ 5839.600120]        [<ffffffff81003c9c>] do_syscall_64+0x6c/0x1e0
>> > [ 5839.606836]        [<ffffffff817d4a3f>] return_from_SYSCALL_64+0x0/0x7a
>> > [ 5839.614231]
>>
>> So here is IMO the real culprit: do_unlinkat() grabs fs freeze protection
>> through mnt_want_write(), we grab also i_rwsem in do_unlinkat() in
>> I_MUTEX_PARENT class a bit after that and further down in vfs_unlink() we
>> grab i_rwsem for the unlinked inode itself in default I_MUTEX class. Then
>> in ovl_want_write() we grab freeze protection again, but this time for the
>> upper filesystem. That establishes sb_writers (overlay) -> I_MUTEX_PARENT
>> (overlay) -> I_MUTEX (overlay) -> sb_writers (FS-A) lock ordering
>> (we maintain locking classes per fs type so that's why I'm showing fs type
>> in parenthesis).
>>
>> Now this nesting is nasty because once you add locks that are not tracked
>> per fs type into the mix, you get cycles. In this case we've got
>> seq_file->lock and cred_guard_mutex into the mix - the splice path is
>> doing sb_writers (FS-A) -> seq_file->lock -> cred_guard_mutex (splicing
>> from seq_file into the real filesystem). Exec path further establishes
>> cred_guard_mutex -> I_MUTEX (overlay) which closes the full cycle:
>>
>> sb_writers (FS-A) -> seq_file->lock -> cred_guard_mutex -> i_mutex
>> (overlay) -> sb_writers (FS-A)
>>
>> If I analyzed the lockdep trace, this looks like a real (although remote)
>> deadlock possibility. Miklos?

Yeah, you can leave out seq_file->lock, the chain can be made up from
just 3 parts:

unlink : i_mutex(ov) -> sb_writers(fs-a)
splice: sb_writers(fs-a) ->cred_guard_mutex (though proc_tgid_io_accounting)
exec:  cred_guard_mutex -> i_mutex(ov)

None of those are incorrect, but the cred_guard_mutex usage is also
pretty weird: taken outside path lookup as well as inside ->read() in
proc.

Doesn't look a serious worry in practice, I don't think anybody would
trigger the actual deadlock in a 1000years (an fs freeze is needed at
just the right moment in addition to the above, very unlikely chain).

Thanks,
Miklos

  reply index

Thread overview: 101+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20160908235521.GL2356@ZenIV.linux.org.uk>
     [not found] ` <20160909015324.GD30056@dastard>
     [not found]   ` <CA+55aFzohsUXj_3BeFNr2t50Wm=G+7toRDEz=Tk7VJqP3n1hXQ@mail.gmail.com>
     [not found]     ` <CA+55aFxrqCng2Qxasc9pyMrKUGFjo==fEaFT1vkH9Lncte3RgQ@mail.gmail.com>
     [not found]       ` <20160909023452.GO2356@ZenIV.linux.org.uk>
     [not found]         ` <CA+55aFwHQMjO4-vtfB9-ytc=o+DRo-HXVGckvXLboUxgpwb7_g@mail.gmail.com>
     [not found]           ` <20160909221945.GQ2356@ZenIV.linux.org.uk>
     [not found]             ` <CA+55aFzTOOB6oEVaaGD0N7Uznk-W9+ULPwzsxS_L_oZqGVSeLA@mail.gmail.com>
     [not found]               ` <20160914031648.GB2356@ZenIV.linux.org.uk>
     [not found]                 ` <20160914133925.2fba4629@roar.ozlabs.ibm.com>
2016-09-18  5:33                   ` xfs_file_splice_read: possible circular locking dependency detected Al Viro
2016-09-19  3:08                     ` Nicholas Piggin
2016-09-19  6:11                       ` Al Viro
2016-09-19  7:26                         ` Nicholas Piggin
     [not found]                 ` <CA+55aFznQaOWoSMNphgGJJWZ=8-odrc0DAUMzfGPQe+_N4UgNA@mail.gmail.com>
     [not found]                   ` <20160914042559.GC2356@ZenIV.linux.org.uk>
     [not found]                     ` <20160917082007.GA6489@ZenIV.linux.org.uk>
     [not found]                       ` <20160917190023.GA8039@ZenIV.linux.org.uk>
2016-09-18 19:31                         ` skb_splice_bits() and large chunks in pipe (was " Al Viro
2016-09-18 20:12                           ` Linus Torvalds
2016-09-18 22:31                             ` Al Viro
2016-09-19  0:18                               ` Linus Torvalds
2016-09-23 19:00                         ` [RFC][CFT] splice_read reworked Al Viro
2016-09-23 19:01                           ` [PATCH 01/11] fix memory leaks in tracing_buffers_splice_read() Al Viro
2016-09-23 19:02                           ` [PATCH 02/11] splice_to_pipe(): don't open-code wakeup_pipe_readers() Al Viro
2016-09-23 19:02                           ` [PATCH 03/11] splice: switch get_iovec_page_array() to iov_iter Al Viro
2016-09-23 19:03                           ` [PATCH 04/11] splice: lift pipe_lock out of splice_to_pipe() Al Viro
2016-09-23 19:45                             ` Linus Torvalds
2016-09-23 20:10                               ` Al Viro
2016-09-23 20:36                                 ` Linus Torvalds
2016-09-24  3:59                                   ` Al Viro
2016-09-24 17:29                                     ` Al Viro
2016-09-27 15:38                                       ` Nicholas Piggin
2016-09-27 15:53                                       ` Chuck Lever
2016-09-24  3:59                                   ` [PATCH 04/12] " Al Viro
2016-09-26 13:35                                     ` Miklos Szeredi
2016-09-27  4:14                                       ` Al Viro
2016-12-17 19:54                                     ` Andreas Schwab
2016-12-18 19:28                                       ` Linus Torvalds
2016-12-18 19:57                                         ` Andreas Schwab
2016-12-18 20:12                                         ` Al Viro
2016-12-18 20:30                                           ` Al Viro
2016-12-18 22:10                                             ` Linus Torvalds
2016-12-18 22:18                                               ` Al Viro
2016-12-18 22:22                                                 ` Linus Torvalds
2016-12-18 22:49                                               ` Andreas Schwab
2016-12-21 18:56                                               ` Andreas Schwab
2016-12-21 19:12                                                 ` Linus Torvalds
2016-09-24  4:00                                   ` [PATCH 06/12] new helper: add_to_pipe() Al Viro
2016-09-26 13:49                                     ` Miklos Szeredi
2016-09-24  4:01                                   ` [PATCH 10/12] new iov_iter flavour: pipe-backed Al Viro
2016-09-29 20:53                                     ` Miklos Szeredi
2016-09-29 22:50                                       ` Al Viro
2016-09-30  7:30                                         ` Miklos Szeredi
2016-10-03  3:34                                           ` [RFC] O_DIRECT vs EFAULT (was Re: [PATCH 10/12] new iov_iter flavour: pipe-backed) Al Viro
2016-10-03 17:07                                             ` Linus Torvalds
2016-10-03 18:54                                               ` Al Viro
2016-09-24  4:01                                   ` [PATCH 11/12] switch generic_file_splice_read() to use of ->read_iter() Al Viro
2016-09-24  4:02                                   ` [PATCH 12/12] switch default_file_splice_read() to use of pipe-backed iov_iter Al Viro
2016-09-23 19:03                           ` [PATCH 05/11] skb_splice_bits(): get rid of callback Al Viro
2016-09-23 19:04                           ` [PATCH 06/11] new helper: add_to_pipe() Al Viro
2016-09-23 19:04                           ` [PATCH 07/11] fuse_dev_splice_read(): switch to add_to_pipe() Al Viro
2016-09-23 19:06                           ` [PATCH 08/11] cifs: don't use memcpy() to copy struct iov_iter Al Viro
2016-09-23 19:08                           ` [PATCH 09/11] fuse_ioctl_copy_user(): don't open-code copy_page_{to,from}_iter() Al Viro
2016-09-26  9:31                             ` Miklos Szeredi
2016-09-23 19:09                           ` [PATCH 10/11] new iov_iter flavour: pipe-backed Al Viro
2016-09-23 19:10                           ` [PATCH 11/11] switch generic_file_splice_read() to use of ->read_iter() Al Viro
2016-09-30 13:32                           ` [RFC][CFT] splice_read reworked CAI Qian
2016-09-30 17:42                             ` CAI Qian
2016-09-30 18:33                               ` CAI Qian
2016-10-03  1:37                                 ` Al Viro
2016-10-03 17:49                                   ` CAI Qian
2016-10-04 17:39                                     ` local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked) CAI Qian
2016-10-04 21:42                                       ` tj
2016-10-05 14:09                                         ` CAI Qian
2016-10-05 15:30                                           ` tj
2016-10-05 15:54                                             ` CAI Qian
2016-10-05 18:57                                               ` CAI Qian
2016-10-05 20:05                                                 ` Al Viro
2016-10-06 12:20                                                   ` CAI Qian
2016-10-06 12:25                                                     ` CAI Qian
2016-10-06 16:11                                                       ` CAI Qian
2016-10-06 17:00                                                         ` Linus Torvalds
2016-10-06 18:12                                                           ` CAI Qian
2016-10-07  9:57                                                           ` Dave Chinner
2016-10-07 15:25                                                             ` Linus Torvalds
2016-10-07  7:08                                                       ` Jan Kara
2016-10-07 14:43                                                         ` CAI Qian
2016-10-07 15:27                                                           ` CAI Qian
2016-10-07 18:56                                                             ` CAI Qian
2016-10-09 21:54                                                               ` Dave Chinner
2016-10-10 14:10                                                                 ` CAI Qian
2016-10-10 20:14                                                                   ` CAI Qian
2016-10-10 21:57                                                                   ` Dave Chinner
2016-10-12 19:50                                                                     ` [bisected] " CAI Qian
2016-10-12 20:59                                                                       ` Dave Chinner
2016-10-13 16:25                                                                         ` CAI Qian
2016-10-13 20:49                                                                           ` Dave Chinner
2016-10-13 20:56                                                                             ` CAI Qian
2016-10-09 21:51                                                           ` Dave Chinner
2016-10-21 15:38                                                         ` [4.9-rc1+] overlayfs lockdep CAI Qian
2016-10-24 12:57                                                           ` Miklos Szeredi [this message]
2016-10-07  9:27                                                     ` local DoS - systemd hang or timeout (WAS: Re: [RFC][CFT] splice_read reworked) Dave Chinner
2016-10-03  1:42                               ` [RFC][CFT] splice_read reworked Al Viro
2016-10-03 14:06                                 ` CAI Qian
2016-10-03 15:20                                   ` CAI Qian
2016-10-03 21:12                                     ` Dave Chinner
2016-10-04 13:57                                       ` CAI Qian
2016-10-03 20:32                                   ` CAI Qian
2016-10-03 20:35                                     ` Al Viro
2016-10-04 13:29                                       ` CAI Qian
2016-10-04 14:28                                         ` Al Viro
2016-10-04 16:21                                           ` CAI Qian
2016-10-04 20:12                                             ` Al Viro
2016-10-05 14:30                                               ` CAI Qian
2016-10-05 16:07                                                 ` Al Viro
2016-09-19  0:22 skb_splice_bits() and large chunks in pipe (was Re: xfs_file_splice_read: possible circular locking dependency detected Al Viro
2016-09-20  9:51 ` Herbert Xu

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