linux-kernel - Re: lockdep trace from posix timers

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Date:	Mon, 20 Aug 2012 17:56:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Mon, 2012-08-20 at 17:41 +0200, Oleg Nesterov wrote:

> 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.

Right, I used a fake callback_head because it avoided a few special
cases since its a dereferencable pointer.

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

Depends on what you're used to I guess ;-) Both RCU and irq_work are
filo, this seems to be the natural way for single linked lists.

> 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.

I'm not entirely sure I see, not doing the cancel would delay the free
until the executing of key_change_session_keyring()? doing that keyctl()
in an indefinite loop involves going back to userspace, so where's the
resource issue?

Also, I'm not seeing where the FIFO requirement comes from.

> > 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).

depends on what way you look at the list I guess, with a single linked
list there's only one end you can add to in O(1), so we're calling that
the tail?

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

Reversing a (single linked) list is O(n^2).. which is indeed doable for
short lists, but why assume its short?
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