linux-kernel - Re: [PATCH v4 1/3] task_work_add: generic process-context callbacks

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Message-ID: <20120414202708.GA8656@redhat.com>
Date:	Sat, 14 Apr 2012 22:27:08 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexander Gordeev <agordeev@...hat.com>,
	Chris Zankel <chris@...kel.net>,
	David Smith <dsmith@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] task_work_add: generic process-context callbacks

On 04/14, Linus Torvalds wrote:
>
> On Fri, Apr 13, 2012 at 8:39 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >>
> > Once the caller does task_work_add(twork), it no longer "owns" this
> > twork.
> >
> > But, if task_work_cancel() succeeds - you own it again.
>
> *IF* it succeeds.

Sure.

> >> But then you can't allocate it on the stack any more, and have to
> >> allocate it separately.
> >
> > Yes, unless you do task_work_add/cancel(current).
>
> Ok, your argument seems to be that "current" is special, and can not
> race, because the work execution is always synchronous with the task
> it got scheduled on.

Yes, exactly.

> And that whole "run_task_work()" function should *not* take a "task"
> pointer, because it would be horribly horribly wrong to ever run it in
> any context than "current".

And it was task_work_queue(void) initially. But then I decided to
micro-optimize this, the callers already have this task_struct in
the register. And we have other examples like this, say, exit_mm().

However. I agree that it would be more understandable and clean
to use current in task_work_run(void), and percpu_read is cheap.

So I'll remove this argument and send v5 after David reviews 3/3.

Oleg.

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