linux-kernel - Re: [PATCH 1/5] vfork: don't freezer_count() for in-kernel users of CLONE

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Date:	Tue, 19 Feb 2013 16:07:54 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/5] vfork: don't freezer_count() for in-kernel users of CLONE_VFORK

On Sat, Feb 16, 2013 at 9:05 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/16, Mandeep Singh Baines wrote:
>>
>> We don't need to call freezer_do_not_count() for in-kernel users
>> of CLONE_VFORK since exec will get called in bounded time.
>>
>> We don't want to call freezer_count() for in-kernel users because
>> they may be holding locks. freezer_count() calls try_to_freeze().
>> We don't want to freeze an in-kernel user because it may be
>> holding locks.
>
> I can only repeat my question ;)
>
>         Who? We should not do this anyway. And __call_usermodehelper() doesn't
>         afaics.
>
>         OK, its caller (process_one_work) does lock_map_acquire() for debugging
>         purposes, this can "confuse" print_held_locks_bug(). But this thread is
>         PF_NOFREEZE ?
>
> Previously this was needed to suppress the false positive. Now that 2/5
> checks PF_NOFREEZE, why do we need this change?
>

After applying the PF_NOFREEZE check, I still get the following:

[    1.001030] =======================================
[    1.001039] [ BUG: lock held while trying to freeze! ]
[    1.001048] 3.4.0 #24 Not tainted
[    1.001053] ---------------------------------------
[    1.001060] kworker/u:0/5 is exiting with locks still held!
[    1.001068] 2 locks held by kworker/u:0/5:
[    1.001073]  #0:  (khelper){.+.+.+}, at: [<8103896f>]
process_one_work+0x108/0
x2ee
[    1.001095]  #1:  ((&sub_info->work)){+.+.+.}, at: [<8103896f>]
process_one_wo
rk+0x108/0x2ee
[    1.001111]
[    1.001113] stack backtrace:
[    1.001119] Pid: 5, comm: kworker/u:0 Not tainted 3.4.0 #24
[    1.001124] Call Trace:
[    1.001135]  [<81025bd6>] ? console_unlock+0x17a/0x18b
[    1.001146]  [<8105d68e>] debug_check_no_locks_held+0x82/0x8a
[    1.001156]  [<8102493f>] do_fork+0x20d/0x2ac
[    1.001167]  [<810366cb>] ? call_usermodehelper_setup+0x8c/0x8c
[    1.001177]  [<81008310>] kernel_thread+0x7a/0x82
[    1.001186]  [<810366cb>] ? call_usermodehelper_setup+0x8c/0x8c
[    1.001198]  [<814b45dc>] ? common_interrupt+0x3c/0x3c
[    1.001208]  [<810365e8>] __call_usermodehelper+0x3b/0x71
[    1.001216]  [<810389ce>] process_one_work+0x167/0x2ee
[    1.001226]  [<810365ad>] ? call_usermodehelper_freeinfo+0x1e/0x1e
[    1.001235]  [<81038dbc>] worker_thread+0xbd/0x18b
[    1.001244]  [<81038cff>] ? rescuer_thread+0x184/0x184
[    1.001254]  [<8103c636>] kthread+0x77/0x7c
[    1.001264]  [<8103c5bf>] ? kthread_freezable_should_stop+0x4a/0x4a
[    1.001273]  [<814b45e2>] kernel_thread_helper+0x6/0x10

Regards,
Mandeep

>> @@ -722,9 +722,11 @@ static int wait_for_vfork_done(struct task_struct *child,
>>  {
>>       int killed;
>>
>> -     freezer_do_not_count();
>> +     if (!(current->flags & PF_KTHREAD))
>> +             freezer_do_not_count();
>
> If I missed something and we really need this, imho this needs a comment.
>
> Oleg.
>
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