linux-kernel - Re: [PATCH 1/5] irq_work: Let arch tell us if it can raise irq work

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Message-ID: <20140513211503.GG13828@localhost.localdomain>
Date:	Tue, 13 May 2014 23:15:05 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] irq_work: Let arch tell us if it can raise irq work

On Tue, May 13, 2014 at 10:48:02PM +0200, Peter Zijlstra wrote:
> On Tue, May 13, 2014 at 09:33:29PM +0200, Frederic Weisbecker wrote:
> > On Tue, May 13, 2014 at 07:09:42PM +0200, Peter Zijlstra wrote:
> > > On Tue, May 13, 2014 at 04:38:37PM +0200, Frederic Weisbecker wrote:
> > > > We prepare for executing the full nohz kick through an irq work. But
> > > > if we do this as is, we'll run into conflicting tick locking: the tick
> > > > holds the hrtimer lock and the nohz kick may do so too.
> > > 
> > > It does? How does the tick end up holding that lock?
> > > 
> > > Normal hrtimer callbacks run without holding the hrtimer lock -- I made
> > > it so.
> > > 
> > > This means tick_sched_timer() is called without hrtimer lock, and I
> > > don't see it taking it anywhere in tick_sched_do_timer() or
> > > tick_sched_handle().
> > 
> > Check hrtimer_interrupt(), it takes the per cpu base->lock.
> 
> check __run_hrtimer() which drops base->lock over calling ->function.

Oh! I had lockdep splats a few days ago. But I think I worked too many hours
on it and eventually developed some brainfarted pet assumptions all along :-(

It was probably due to some other mistakes of mine. Ok, lets try again.

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