linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Date:	Thu, 14 Aug 2014 21:37:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
	seqlock

On 08/14, Rik van Riel wrote:
>
> On 08/14/2014 02:15 PM, Oleg Nesterov wrote:
> > On 08/14, Rik van Riel wrote:
> >>
> >> On 08/14/2014 12:12 PM, Oleg Nesterov wrote:
> >>>
> >>> Or you can expand the scope of write_seqlock/write_sequnlock, so that
> >>> __unhash_process in called from inside the critical section. This looks
> >>> simpler at first glance.
> >>
> >> The problem with that is that wait_task_zombie() calls
> >> thread_group_cputime_adjusted() in that if() branch, and
> >> that code ends up taking the seqlock for read...
> >
> > Not sure I understand... This modifies parent->signal->c* counters,
> > and obviously the exiting thread is not the member of parent's thread
> > group, so thread_group_cputime_adjusted(parent) can never account the
> > exiting child twice simply because it won't see it?
>
> You are right, the tree of processes only goes one way,
> so there should be no deadlock in taking psig->stats_lock
> and having thread_group_cputime_adjusted take sig->stats_lock
> for read within that section.
>
> However, it might need some lockdep annotation to keep
> lockdep from thinking we might the same lock recursively :)

But wait_task_zombie() can (and should) call
thread_group_cputime_adjusted(zombie_child) outside of parent's ->siglock
or ->stats_lock so this this should be safe.

Oleg.

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