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SubjectRe: [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead
On Thu, Apr 08, 2021 at 12:56:01PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 09:24:12PM +0100, Mel Gorman wrote:
> > Why local_lock? PREEMPT_RT considers the following sequence to be unsafe
> > as documented in Documentation/locking/locktypes.rst
> >
> > local_irq_disable();
> > raw_spin_lock(&lock);
>
> Almost, the above is actually OK on RT. The problematic one is:
>
> local_irq_disable();
> spin_lock(&lock);
>
> That doesn't work on RT since spin_lock() turns into a PI-mutex which
> then obviously explodes if it tries to block with IRQs disabled.
>
> And it so happens, that's exactly the one at hand.

Ok, I completely messed up the leader because it was local_irq_disable()
+ spin_lock() that I was worried about. Once the series is complete,
it is replated with

local_lock_irq(&lock_lock)
spin_lock(&lock);

According to Documentation/locking/locktypes.rst, that should be safe.
I'll rephrase the justification.

--
Mel Gorman
SUSE Labs

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