linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <1345476436.23018.54.camel@twins>
Date:	Mon, 20 Aug 2012 17:27:16 +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 17:10 +0200, Peter Zijlstra wrote:
> On Mon, 2012-08-20 at 16:59 +0200, Oleg Nesterov wrote:
> > On 08/20, Peter Zijlstra wrote:
> > >
> > >  task_work_cancel(struct task_struct *task, task_work_func_t func)
> > >  {
> > > ...
> > > +
> > > +again:
> > > +	workp = &task->task_works;
> > > +	work = *workp;
> > > +	while (work) {
> > > +		if (work->func == func) {
> > 
> > But this all can race with task_work_run() if task != current.
> > 
> > This "work" can be freed/reused. And it should only return !NULL
> > if twork->func() was not called.
> 
> Ah, because we could be iterating the list after the xchg done by run.

I guess we could steal the entire list and requeue it afterwards and
lift TIF_NOTIFY_RESUME along with it.. but I can't say that's pretty.




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