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Message-ID: <1345478620.23018.71.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>,
        dhowells <dhowells@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Date: Mon, 20 Aug 2012 18:03:40 +0200
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On Mon, 2012-08-20 at 17:58 +0200, Oleg Nesterov wrote:
> On 08/20, Peter Zijlstra wrote:
> >
> > On Mon, 2012-08-20 at 17:32 +0200, Oleg Nesterov wrote:
> >
> > > > 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().
> >
> > So we could put that spinlock back around cancel and run and leave add
> > lockless. That'd solve my immediate problem but its not something I'm
> > proud of.
> 
> Which problem?

/me doing task_work_add() from under rq->lock..

> We can probably use bit_spin_lock() and avoid ->pi_lock.

tglx will kill us both for even thinking of bit-spinlocks.

> Of course, we can add the new lock into task_struct, but this is not nice.

If we can limit the lock to cancel/run I'm ok.