Message-ID: <CAHQdGtQhurcOsAii7PYNZwEL4Xm-8a2CKBkS4OhrS0FhaAX3Aw@mail.gmail.com>
Date:	Mon, 2 Jun 2014 20:59:21 -0400
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Jeff Layton <jeff.layton@...marydata.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Linux Kernel mailing list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: nfs4_do_reclaim lockdep pop in v3.15.0-rc1

On Mon, Jun 2, 2014 at 6:49 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Mon, Jun 2, 2014 at 3:42 PM, Trond Myklebust
> <trond.myklebust@...marydata.com> wrote:
>> On Mon, Jun 2, 2014 at 6:12 PM, John Stultz <john.stultz@...aro.org> wrote:
>>> On Mon, Jun 2, 2014 at 9:02 AM, Trond Myklebust
>>> <trond.myklebust@...marydata.com> wrote:
>>>> On Mon, Jun 2, 2014 at 10:49 AM, Jeff Layton
>>>> <jeff.layton@...marydata.com> wrote:
>>>>> I've been working on the patchset to break up the client_mutex in nfsd.
>>>>> While doing some debugging, I had mounted my kernel git tree with
>>>>> NFSv4.1, and was running crash on the vmlinux image in it.
>>>>>
>>>>> A little while later, I saw the following lockdep inversion pop.
>>>>> Unfortunately, I couldn't get the whole log, but I think it's enough to
>>>>> show that there's a potential problem?
>>>>>
>>>>> I've not had time to give it a hard look yet, but thought I'd post it
>>>>> here in the hopes that it might look familiar to someone:
>>>>>
>>>>> [ 2581.104687] ======================================================
>>>>> [ 2581.104716] [ INFO: possible circular locking dependency detected ]
>>>>> [ 2581.104716] 3.15.0-rc1.jlayton.1+ #2 Tainted: G           OE
>>>>> [ 2581.104716] -------------------------------------------------------
>>>>> [ 2581.104716] 2001:470:8:d63:/5622 is trying to acquire lock:
>>>>> [ 2581.104716]  (&(&sp->so_lock)->rlock){+.+...}, at: [<ffffffffa036dbdd>] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4]
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] but task is already holding lock:
>>>>> [ 2581.104716]  (&sp->so_reclaim_seqcount){+.+...}, at: [<ffffffffa036e5fe>] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] which lock already depends on the new lock.
>>>>> [ 2581.104716]
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] the existing dependency chain (in reverse order) is:
>>>>> [ 2581.104716]
>>>>> -> #1 (&sp->so_reclaim_seqcount){+.+...}:
>>>>> [ 2581.104716]        [<ffffffff810f9aa2>] lock_acquire+0xa2/0x1d0
>>>>> [ 2581.104716]        [<ffffffffa036d8b0>] nfs4_do_reclaim+0x290/0x7f0 [nfsv4]
>>>>> [ 2581.104716]        [<ffffffffa036e5fe>] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]        [<ffffffff810c260f>] kthread+0xff/0x120
>>>>> [ 2581.104716]        [<ffffffff817e6bfc>] ret_from_fork+0x7c/0xb0
>>>>> [ 2581.104716]
>>>>> -> #0 (&(&sp->so_lock)->rlock){+.+...}:
>>>>> [ 2581.104716]        [<ffffffff810f919f>] __lock_acquire+0x1b8f/0x1ca0
>>>>> [ 2581.104716]        [<ffffffff810f9aa2>] lock_acquire+0xa2/0x1d0
>>>>> [ 2581.104716]        [<ffffffff817dbdae>] _raw_spin_lock+0x3e/0x80
>>>>> [ 2581.104716]        [<ffffffffa036dbdd>] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4]
>>>>> [ 2581.104716]        [<ffffffffa036e5fe>] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]        [<ffffffff810c260f>] kthread+0xff/0x120
>>>>> [ 2581.104716]        [<ffffffff817e6bfc>] ret_from_fork+0x7c/0xb0
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] other info that might help us debug this:
>>>>> [ 2581.104716]
>>>>> [ 2581.104716]  Possible unsafe locking scenario:
>>>>> [ 2581.104716]
>>>>> [ 2581.104716]        CPU0                    CPU1
>>>>> [ 2581.104716]        ----                    ----
>>>>> [ 2581.104716]   lock(&sp->so_reclaim_seqcount);
>>>>> [ 2581.104716]                                lock(&(&sp->so_lock)->rlock);
>>>>> [ 2581.104716]                                lock(&sp->so_reclaim_seqcount);
>>>>> [ 2581.104716]   lock(&(&sp->so_lock)->rlock);
>>>>> [ 2581.104716]
>>>>> [ 2581.104716]  *** DEADLOCK ***
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] 1 lock held by 2001:470:8:d63:/5622:
>>>>> [ 2581.104716]  #0:  (&sp->so_reclaim_seqcount){+.+...}, at: [<ffffffffa036e5fe>] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]
>>>>> [ 2581.104716] stack backtrace:
>>>>> [ 2581.104716] CPU: 2 PID: 5622 Comm: 2001:470:8:d63: Tainted: G           OE 3.15.0-rc1.jlayton.1+ #2
>>>>> [ 2581.104716] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>>>>> [ 2581.104716]  0000000000000000 00000000d29e16c4 ffff8800d8d8fba8 ffffffff817d318e
>>>>> [ 2581.104716]  ffffffff8262d5e0 ffff8800d8d8fbe8 ffffffff817ce525 ffff8800d8d8fc40
>>>>> [ 2581.104716]  ffff8800362a8b98 ffff8800362a8b98 0000000000000001 ffff8800362a8000
>>>>> [ 2581.104716] Call Trace:
>>>>> [ 2581.104716]  [<ffffffff817d318e>] dump_stack+0x4d/0x66
>>>>> [ 2581.104716]  [<ffffffff817ce525>] print_circular_bug+0x201/0x20f
>>>>> [ 2581.104716]  [<ffffffff810f919f>] __lock_acquire+0x1b8f/0x1ca0
>>>>> [ 2581.104716]  [<ffffffff813dbe9e>] ? debug_check_no_obj_freed+0x17e/0x270
>>>>> [ 2581.104716]  [<ffffffff810f9aa2>] lock_acquire+0xa2/0x1d0
>>>>> [ 2581.104716]  [<ffffffffa036dbdd>] ? nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffff817dbdae>] _raw_spin_lock+0x3e/0x80
>>>>> [ 2581.104716]  [<ffffffffa036dbdd>] ? nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffffa036dbdd>] nfs4_do_reclaim+0x5bd/0x7f0 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffffa036e5fe>] ? nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffffa036e5fe>] nfs4_run_state_manager+0x7ee/0xc00 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffffa036de10>] ? nfs4_do_reclaim+0x7f0/0x7f0 [nfsv4]
>>>>> [ 2581.104716]  [<ffffffff810c260f>] kthread+0xff/0x120
>>>>> [ 2581.104716]  [<ffffffff810c2510>] ? insert_kthread_work+0x80/0x80
>>>>> [ 2581.104716]  [<ffffffff817e6bfc>] ret_from_fork+0x7c/0xb0
>>>>> [ 2581.104716]  [<ffffffff810c2510>] ? insert_kthread_work+0x80/0x80
>>>>
>>>> OK. So now that lockdep has been added to raw_seqcount_begin() (commit
>>>> 1ca7d67cf5d5a), exactly what are we supposed to use when we DON'T want
>>>> lockdep to "sanity check" our locking here?
>>>
>>> So raw_write_seqcount_*  provides the lockdep-disabled methods.
>>>
>>>> As far as we're concerned, the above check is completely bogus, and
>>>> there is no deadlock. At best it would be a livelock, and it would be
>>>> because the server is rebooting over and over again (in which case the
>>>> client behaviour of retrying is _correct_).
>>>
>>> I've not been able to totally trace the locking path there, but having
>>> a seqlock writes and spinlock ABBA deadlock seems problematic.... at
>>> least at first glance.
>>>
>>> So if I'm reading this right...  nfs4_reclaim_open_state() takes a
>>> spinlock on so_lock, then the write on the so_reclaim_seqcount, then
>>> drops the so_lock and calls nfs4_put_open_state which reaquires the
>>> so_lock.
>>>
>>> And lockdep is worried there may be another thread which called into
>>> nfs4_reclaim_open_state() and took the so_lock while it was
>>> momentarily free, and is blocking waiting on the so_reclaim_seqcount.
>>> This would cause the first threads requisition of the so_lock to
>>> potentially deadlock.
>>>
>>> And your point is that this isn't a concern since no other threads can
>>> call nfs4_reclaim_open_state() or any other code path that acquires
>>> those two locks in order?  If you're going to disable the lockdep
>>> checks here, you might want to make this restriction really clear in a
>>> comment so no one accidentally breaks that rule.
>>>
>>
>> It's a general rule in the NFSv4 client that the only thread that is
>> allowed to call state recovery functions (after a server reboot or a
>> long-lived network partition) is the state manager thread. That
>> applies to _all_ state, not just open state.
>>
>> The so_reclaim_seqcount only exists in order to tell the other threads
>> that they may need to replay file open or file lock requests that have
>> raced with state recovery (because those threads got scheduled out
>> after their RPC calls ran, but before they managed to set up the
>> tracking of the new state). It is basically an edge condition
>> killer...
>
> Would then swapping the acquisition order, so the seqcount is taken
> before the so_lock at the top of nfs4_reclaim_open_state() avoid this
> then, without having to disable lockdep?
>

I can change the write seqcount to use raw_write_seqcount(), but that
doesn't answer the question of why raw_seqcount_begin() is the _only_
object out there with a "raw_" prefix, that doesn't explicitly disable
lockdep checking.

What justifies the inconsistency?

-- 
Trond Myklebust

Linux NFS client maintainer, PrimaryData

trond.myklebust@...marydata.com
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