linux-kernel - Re: lockdep trace from posix timers

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Message-ID: <20120828171236.GA31080@redhat.com>
Date:	Tue, 28 Aug 2012 19:12:36 +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 08/28, Oleg Nesterov wrote:
>
> On 08/28, Peter Zijlstra wrote:
> >
> > +again:
> > +	workp = &task->task_works;
> > +	work = *workp;
> > +	while (work) {
> > +		if (work->func == func) {
>
> But you can't dereference this pointer. Without some locking this
> can race with another task_work_cancel() or task_work_run(), this
> work can be free/unmapped/reused.
>
> > +			if (cmpxchg(workp, work, work->next) == work)
> > +				return work;
>
> Or this can race with task_work_cancel(work) + task_work_add(work).
> cmpxchg() can succeed even if work->func is already different.

Even simpler, this can race with another task_work_cancel() which
is going to remove work->next.

Oleg.

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