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

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH] x86/traps: use conditional_{cli,sti} in preempt_conditinal_{cli_sti}
Date	2016-01-16 23:50 +0100
Message-ID	<qRHPs-2hu-9@gated-at.bofh.it> (permalink)
References	<qRyMa-4L7-5@gated-at.bofh.it> <qREoy-ag-7@gated-at.bofh.it>
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On Sat, Jan 16, 2016 at 11:06 AM, Borislav Petkov <bp@suse.de> wrote:
> 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@fastmail.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@gmail.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.
>

I agree.  Every time I look at those helpers I wonder what the condition is.

Also, "preempt_conditional_sti" meaning "enable interrupts if they
were enabled but *don't* enable preemption" just seems backwards.

--Andy

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[PATCH] x86/traps: use conditional_{cli,sti} in preempt_conditinal_{cli_sti} Alexander Kuleshov <kuleshovmail@gmail.com> - 2016-01-16 14:10 +0100
  Re: [PATCH] x86/traps: use conditional_{cli,sti} in  preempt_conditinal_{cli_sti} Borislav Petkov <bp@suse.de> - 2016-01-16 20:10 +0100
    Re: [PATCH] x86/traps: use conditional_{cli,sti} in preempt_conditinal_{cli_sti} Andy Lutomirski <luto@amacapital.net> - 2016-01-16 23:50 +0100

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