linux-kernel - Re: lockdep trace from posix timers

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Date:	Mon, 20 Aug 2012 17:32:25 +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 17:10 +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..

We can't. This can race with exit_task_work(). And this can break
fput(), the task can return to the userspace without __fput().

> but I can't say that's pretty.

Yes ;)

Oleg.

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