linux-kernel - Re: [patch V2 00/24] cpu/hotplug: Convert get_online_cpus() to a percpu

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Date:   Wed, 26 Apr 2017 11:32:36 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:     Sebastian Siewior <bigeasy@...utronix.de>, catalin.marinas@....com,
        will.deacon@....com, Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [patch V2 00/24] cpu/hotplug: Convert get_online_cpus() to a
 percpu_rwsem

On Wed, Apr 26, 2017 at 10:40:47AM +0100, Suzuki K Poulose wrote:
> On 26/04/17 09:59, Mark Rutland wrote:

> >We only call cpus_set_cap() in the secondary boot path, where we know
> >that the rwsem is held by the thread orchestrating the onlining. Thus,
> >we can use the new static_branch_enable_cpuslocked() in cpus_set_cap(),
> >avoiding the above.
> 
> Correction, we could call cpus_set_cap() from setup_cpu_features()
> for updating the system global capabilities, where we may not hold the
> cpu_hotplug_lock. 

Argh, yes, I missed that when scanning.

> So we could end up calling static_branch_enable_cpuslocked()
> without actually holding the lock. Should we do a cpu_hotplug_begin/done in
> setup_cpu_feature_capabilities ? I agree it doesn't look that nice. Thoughts ?

I agree that's hideous, but it looks like the only choice given the
hotplug rwsem cahnges. :/

I can spin a v2 with that and the typo fixes.

Thanks,
Mark.

> 
> Suzuki
> 
> >		
> >Signed-off-by: Mark Rutland <mark.rutland@....com>
> >Reported-by: Catalin Marinas <catalin.marinas@....com>
> >Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> >Cc: Will Deacon <will.deacon@....com>
> >Cc: Suzuki Poulose <suzuki,poulose@....com>
> >---
> > arch/arm64/include/asm/cpufeature.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> >index f31c48d..349b5cd 100644
> >--- a/arch/arm64/include/asm/cpufeature.h
> >+++ b/arch/arm64/include/asm/cpufeature.h
> >@@ -145,7 +145,7 @@ static inline void cpus_set_cap(unsigned int num)
> > 			num, ARM64_NCAPS);
> > 	} else {
> > 		__set_bit(num, cpu_hwcaps);
> >-		static_branch_enable(&cpu_hwcap_keys[num]);
> >+		static_branch_enable_cpuslocked(&cpu_hwcap_keys[num]);
> > 	}
> > }
> >
> >
>