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Message-ID: <1345479590.23018.75.camel@twins>
Subject: Re: lockdep trace from posix timers
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
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Date: Mon, 20 Aug 2012 18:19:50 +0200
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On Mon, 2012-08-20 at 18:10 +0200, Oleg Nesterov wrote:
> task->task_works points to the last node in the circular single-linked list,
> task_work_add() adds the new element after the last one and updates
> task->task_works. This is O(1).

Agreed, the way I was looking at that is: ->task_works points to the
head and we put a new one in front, that too 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.
> >
> > Reversing a (single linked) list is O(n^2)..
> 
> Hmm. This is O(n). You can simply iterate over this list once, changing
> the ->next pointer to point back. 

OK, I'm going to stop and step away from the computer now.. clearly I
more than useless today :/

But yeah.. that could be done.

Anyway, would taking ->pi_lock over _cancel and _run suffice?