linux-kernel - Re: [PATCH] x86/traps: use conditional_{cli,sti} in preempt_conditinal_{cli

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Date:	Sat, 16 Jan 2016 20:06:47 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Alexander Kuleshov <kuleshovmail@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andy Lutomirski <luto@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Wang Nan <wangnan0@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/traps: use conditional_{cli,sti} in
 preempt_conditinal_{cli_sti}

On Sat, Jan 16, 2016 at 06:58:07PM +0600, Alexander Kuleshov wrote:
> The 3d2a71a596bd9 commit (x86, traps: converge do_debug handlers by
> Alexander van Heukelum <heukelum@...tmail.fm>) introduces two functions:
> preempt_conditional_sti/cli() which are enables/disables interrupts
> depends on state of the interrupt enable flag and increments/decrements
> the preempt counter.
> 
> In the same time arch/x86/kernel/traps.c defines two similar inline
> functions: conditional_{sti,cli} which are do the same, but without
> touch of the preempt counter. Let's use these functions in the
> preempt_conditional_{sti,cli} instead of duplication of 'if' statemets.
> 
> Signed-off-by: Alexander Kuleshov <kuleshovmail@...il.com>
> ---
>  arch/x86/kernel/traps.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index ade185a..30ec8fa 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -92,8 +92,7 @@ static inline void conditional_sti(struct pt_regs *regs)
>  static inline void preempt_conditional_sti(struct pt_regs *regs)
>  {
>  	preempt_count_inc();
> -	if (regs->flags & X86_EFLAGS_IF)
> -		local_irq_enable();
> +	conditional_sti(regs);

What I would do is kill both preempt_conditional_sti() and
preempt_conditional_cli() instead. Why?

Because call sites become more readable:

	preempt_disable();

	if (regs->flags & X86_EFLAGS_IF)
		local_irq_enable();

and

	if (regs->flags & X86_EFLAGS_IF)
		local_irq_disable();

	preempt_enable_no_resched();

Those preempt_* variants are just silly.

Yes, I'd delete conditional_cli() because nothing uses it. And since I'm
deleting crap, I'd delete conditional_sti() too because it's naming is
ugly. You have to know that "STI" means Set Interrupt Flag. Now, if it
were called

	cond_local_irq_enable()

that would be better.

So yeah, IMO, the most understandable variant would be:

	preempt_disable();
	cond_local_irq_enable();

and the opposing pair:

	cond_local_irq_disable();
	preempt_enable_no_resched();

And *that* is actually understandable at a quick glance.

But that's just me.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--