linux-kernel - Re: [PATCH] copy_process: cosmetic ->ioprio tweak

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Date:	Tue, 22 Aug 2006 22:27:07 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...l.org>
Cc:	Jens Axboe <axboe@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] copy_process: cosmetic ->ioprio tweak

On 08/21, Andrew Morton wrote:
>
> On Sun, 20 Aug 2006 18:53:21 +0400
> Oleg Nesterov <oleg@...sign.ru> wrote:
> 
> > copy_process:
> > // holds tasklist_lock + ->siglock
> >        /*
> >         * inherit ioprio
> >         */
> >        p->ioprio = current->ioprio;
> > 
> > Why? ->ioprio was already copied in dup_task_struct().
> 
> It might just be a thinko.
> 
> > I guess this is needed
> > to ensure that the child can't escape sys_ioprio_set(IOPRIO_WHO_{PGRP,USER}),
> > yes?
> 
> How could the child escape that if this assignment was not present?

It is possible that sys_ioprio_set(IOPRIO_WHO_PGRP) was called after
copy_process() already did dup_task_struct(), but before it takes
tasklist_lock. Documentation/block/ioprio.txt doesn't say should
ioprio_set() be "atomic" or not. If not, we can kill this line, and
(more importantly) drop tasklist_lock in fs/ioprio.c

Oleg.

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