Re: nfs4_do_reclaim lockdep pop in v3.15.0-rc1

[John Stultz]

On Mon, Jun 2, 2014 at 9:02 AM, Trond Myklebust
<trond.myklebust@xxxxxxxxxxxxxxx> wrote:
> On Mon, Jun 2, 2014 at 10:49 AM, Jeff Layton
> <jeff.layton@xxxxxxxxxxxxxxx> 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.

thanks
-john
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