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From: Thomas Gleixner <tglx@kernel.org>
To: Boqun Feng <boqun@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
 linux-tip-commits@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] locking: Revert switching guards to
 _irq_{disable,enable}()
In-Reply-To: <87v78wezid.ffs@fw13>
References: <20260804161447.84806-8-boqun@kernel.org>
 <178635226387.442315.3868294476114711805.tip-bot2@tip-bot2>
 <20260824104704.GA4121339@noisy.programming.kicks-ass.net>
 <20260824105523.GA4121620@noisy.programming.kicks-ass.net>
 <aozw_7Txxaq2qE7c@tardis.local> <877bldhkmq.ffs@fw13>
 <ao4lO7U7L-7Zice1@tardis.local> <87v78wezid.ffs@fw13>
Date: Thu, 27 Aug 2026 22:29:10 +0200
Message-ID: <875x0vfgtl.ffs@fw13>
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On Thu, Aug 27 2026 at 10:30, Thomas Gleixner wrote:
> On Tue, Aug 25 2026 at 16:28, Boqun Feng wrote:
> So you can get away with:
>
> raw_local_irq_save(flags)
> {
> 	flags = count;
> 	if (!count)
>         	arch_local_irq_disable();
>         count++;
> }
>
> raw_local_irq_restore(flags)
> {
> 	if (!(count = flags))
>         	arch_local_irq_enable();
> }
>
> raw_local_irq_disable()
> {
>         arch_local_irq_disable();
>         count = 1;
> }
>
> raw_local_irq_enable()
> {
>         count = 0;
>         arch_local_irq_enable();
> }

Actually it can be done way simpler because the nasty case of

         scoped_guard(lock_irqsave, lock) {
         	unlock_irq(lock);
                lock_irq(lock);
         }

is only valid for a single lock guard, because if it's nested then the
outer lock would lose the interrupt disabled protection. Anything else
would be a bug on its own and would have long ago blown up in our face.

So we can completely ignore flags.

raw_local_irq_save(flags)
{
 	if (!count)
         	arch_local_irq_disable();
        count++;
}

raw_local_irq_restore(flags)
{
 	if (!--count)
         	arch_local_irq_enable();
}

raw_local_irq_disable()
{
         arch_local_irq_disable();
         count++;
}

raw_local_irq_enable()
{
         count--;
         arch_local_irq_enable();
}

with a copious amount of debug machinery to catch any oddballs.

Also note that this never uses irqsave/restore because historically that
has been way slower than CLI/STI.

A decade+ ago this used to be up to 30%, but micro architectures
optimized for it. Still on a SKL it's ~14% and on a Zen3 ~8% slower.

Thanks,

        tglx