linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <20120820154154.GB20258@redhat.com>
Date:	Mon, 20 Aug 2012 17:41:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	rostedt <rostedt@...dmis.org>, dhowells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: lockdep trace from posix timers

On 08/20, Peter Zijlstra wrote:
>
> On Mon, 2012-08-20 at 17:05 +0200, Oleg Nesterov wrote:
> >
> > Yes, we can add the explicit argument to __task_work_run(), but it can
> > check PF_EXITING instead, this looks simpler to me.
>
> I guess we could.. but I thought the explicit callback was simpler ;-)

I won't insist. The patch I sent uses PF_EXITING and the fake
"struct callback_head* TWORK_EXITED", but this looks almost the same.

> > Note also your patch breaks fifo, but this is fixable.
>
> Why do you care about the order?

IMHO, this is just more natural.

For example. keyctl_session_to_parent() does _cancel only to protect
from exploits doing keyctl(KEYCTL_SESSION_TO_PARENT) in an endless
loop. It could simply do task_work_add(), but in this case we need
fifo for correctness.

> Iterating a single linked queue in fifo
> seems more expensive than useful.

Currently the list is fifo (we add to the last element), this is O(1).

But the list should be short, we can reverse it in _run() if we change
task_work_add() to add to the head.

Oleg.

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