linux-kernel - Re: [GIT pull] x86/urgent for v6.11-rc3

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Message-ID: <87le12rw26.ffs@tglx>
Date: Mon, 12 Aug 2024 10:21:05 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [GIT pull] x86/urgent for v6.11-rc3

On Sun, Aug 11 2024 at 11:07, Linus Torvalds wrote:
> On Sun, 11 Aug 2024 at 06:58, Thomas Gleixner <tglx@...utronix.de> wrote:
> Our static key code is pretty confusing, but we basically have
>
>  - virt_spin_lock_key is now a "struct static_key_false", which means
> that we consider the virt case the unlikely case.
>
>    I agree whole-heartedly, because it's going to be the slow case
> anyway, so this is good.
>
>  - that means that 'static_branch_likely()' will generate a branch
> (because the key is marked unlikely0
>
> Isn't this wrong? So instead of falling through to the native
> qspinlock case, we will branch to it, and we fall through to the
> virt-spinlock case?
>
> So i think that static_branch_likely() should have been changed to a
> static_branch_unlikely() too, but it's possible that I've just
> confused myself.

You are right. It creates a branch for the !virt_lock case.

> Anyway, somebody should double-check me.
>
> I doubt it actually matters, since I think this all is fundamentally
> just in the slow-path, so the "do a branch or a no-op" is likely
> entirely in the noise even if I followed the code right. But it looked
> off to me.

It is off and yes it won't matter much in the slowpath maze.

Thanks

        tglx