linux-kernel - Re: [PATCH 04/36] cpuidle,intel_idle: Fix CPUIDLE_FLAG_IRQ

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Date:   Mon, 13 Jun 2022 10:44:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
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Subject: Re: [PATCH 04/36] cpuidle,intel_idle: Fix CPUIDLE_FLAG_IRQ_ENABLE

On Thu, Jun 09, 2022 at 04:49:21PM -0700, Jacob Pan wrote:
> Hi Peter,
> 
> On Wed, 08 Jun 2022 16:27:27 +0200, Peter Zijlstra <peterz@...radead.org>
> wrote:
> 
> > Commit c227233ad64c ("intel_idle: enable interrupts before C1 on
> > Xeons") wrecked intel_idle in two ways:
> > 
> >  - must not have tracing in idle functions
> >  - must return with IRQs disabled
> > 
> > Additionally, it added a branch for no good reason.
> > 
> > Fixes: c227233ad64c ("intel_idle: enable interrupts before C1 on Xeons")
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  drivers/idle/intel_idle.c |   48
> > +++++++++++++++++++++++++++++++++++----------- 1 file changed, 37
> > insertions(+), 11 deletions(-)
> > 
> > --- a/drivers/idle/intel_idle.c
> > +++ b/drivers/idle/intel_idle.c
> > @@ -129,21 +137,37 @@ static unsigned int mwait_substates __in
> >   *
> >   * Must be called under local_irq_disable().
> >   */
> nit: this comment is no long true, right?

It still is, all the idle routines are called with interrupts disabled,
but must also exit with interrupts disabled.

If the idle method requires interrupts to be enabled, it must be sure to
disable them again before returning. Given all the RCU/tracing concerns
it must use raw_local_irq_*() for this though.