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Date: Mon, 20 Aug 2012 17:32:25 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
        dhowells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: lockdep trace from posix timers
Message-ID: <20120820153225.GA20258@redhat.com>
References: <20120724203613.GA9637@redhat.com> <1345140478.29668.54.camel@twins> <20120817151447.GA7918@redhat.com> <1345446957.23018.14.camel@twins> <1345463081.23018.34.camel@twins> <20120820145919.GB18499@redhat.com> <1345475434.23018.48.camel@twins> <1345476436.23018.54.camel@twins>
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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.