linux-kernel - Re: [PATCH 1/2] drivers: cpuidle: remove stale irq disabling call in cpuidle_enter

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Date:	Wed, 25 Feb 2015 14:39:17 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH 1/2] drivers: cpuidle: remove stale irq disabling call in
 cpuidle_enter_freeze()

On Wed, Feb 25, 2015 at 02:13:23PM +0000, Daniel Lezcano wrote:
> On 02/24/2015 06:58 PM, Lorenzo Pieralisi wrote:
> > On return from cpuidle_enter_freeze() irqs are re-enabled by the function
> > caller (ie cpuidle_idle_call) in the idle loop. This patch removes a stale
> > local_irq_disable() call and its stale comment in cpuidle_enter_freeze(),
> > since they disagree and do not serve a useful purpose.
> >
> > Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> > Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> > ---
> >   drivers/cpuidle/cpuidle.c | 3 ---
> >   1 file changed, 3 deletions(-)
> >
> > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> > index 4d53458..f47edc6c 100644
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -144,9 +144,6 @@ void cpuidle_enter_freeze(void)
> >   		cpuidle_enter(drv, dev, index);
> >   	else
> >   		arch_cpu_idle();
> > -
> > -	/* Interrupts are enabled again here. */
> > -	local_irq_disable();
> >   }
> 
> Hmm, I think Rafael added this prevent lockdep to raise a warning.

Ok, so the comment is there to say "at this point of execution IRQs
are enabled", it does not refer to local_irq_disable() call effects,
that's misleading and not necessarily nice, at least it should
be explained.

> Otherwise, cpuidle_enter or arch_cpu_idle enables the irq again and then 
> when exiting the cpu_idle_call, we enable them again, so leading to a 
> lockdep WARN in trace_hardirqs_on_caller.

Would not it be better to enable irqs in cpuidle_enter_freeze() on
returning from enter_freeze_proper() and remove the local_irq_enable()
call in the cpuidle_idle_call() before jumping to exit_idle ?

> That said, if we have to do this, it may reveal something is wrong in 
> the code.

I just spotted code through inspection, I have to say at the moment it
is not very clear what it is meant to achieve, so I put together this
patch.

Lorenzo
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