extra reference to fl->fl_file, possible regression

[William Dauchy <william@gandi.net>]

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Hello,

We have been testing the two following patches on top of the last 3.14.x.
(they have been queued up for stable releases)

commit db2efec0caba4f81a22d95a34da640b86c313c8e
Author: Jeff Layton <jlayton@poochiereds.net>
Date:   Tue Jun 30 14:12:30 2015 -0400

    nfs: take extra reference to fl->fl_file when running a LOCKU operation

commit feaff8e5b2cfc3eae02cf65db7a400b0b9ffc596
Author: Jeff Layton <jlayton@poochiereds.net>
Date:   Tue May 12 15:48:10 2015 -0400

    nfs: take extra reference to fl->fl_file when running a setlk


It resulted in random instabilities; we are unable to reproduce it reliably for now;
the only trace we got was the one below.

------------[ cut here ]------------
WARNING: CPU: 0 PID: 8 at kernel/rcu/tree.c:2191 rcu_do_batch.isra.51+0x384/0x3d0()
CPU: 0 PID: 8 Comm: rcuc/0 Not tainted 3.14 #1
0000000000000009 ffff88061e5dfd10 ffffffff815f5143 0000000000000000
ffff88061e5dfd48 ffffffff8105eece 000000000000002f ffff880627c0b600
000000000000002f 0000000000000246 0000000000000000 ffff88061e5dfd58
Call Trace:
[<ffffffff815f5143>] dump_stack+0x4d/0x81
[<ffffffff8105eece>] warn_slowpath_common+0x6e/0x90
[<ffffffff8105efd5>] warn_slowpath_null+0x15/0x20
[<ffffffff810bc634>] rcu_do_batch.isra.51+0x384/0x3d0
[<ffffffff810bc42a>] ? rcu_do_batch.isra.51+0x17a/0x3d0
[<ffffffff810bc9ed>] rcu_cpu_kthread+0xed/0x130
[<ffffffff8108aabe>] smpboot_thread_fn+0x18e/0x2e0
[<ffffffff8108a930>] ? in_egroup_p+0x40/0x40
[<ffffffff8108358c>] kthread+0xec/0x110
[<ffffffff810834a0>] ? __kthread_parkme+0x80/0x80
[<ffffffff815fcb39>] ret_from_fork+0x49/0x80
[<ffffffff810834a0>] ? __kthread_parkme+0x80/0x80
---[ end trace 27f9589ec4225b03 ]---
-- 
William

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