Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

[Borislav Petkov]

On Tue, Apr 28, 2015 at 01:38:41PM +0000, Zheng, Lv wrote:
> > -	raw_spin_lock(&ghes_nmi_lock);
> > +	if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> > +		return ret;
> > +
> 
> if (atomic_cmpxchg(&ghes_in_nmi, 0, 1))
> 	return ret;

Ok, now I understand what you mean.

We absolutely want to use atomic_add_unless() because we get to save us
the expensive

	LOCK; CMPXCHG

if the value was already 1. Which is exactly what this patch is trying
to avoid - a thundering herd of cores CMPXCHGing a global variable.

I.e.,

	movl	ghes_in_nmi(%rip), %ecx	# MEM[(const int *)&ghes_in_nmi], c
	cmpl	$1, %ecx	#, c
	je	.L311	#,				<--- exit here if ghes_in_nmi == 1.
	leal	1(%rcx), %edx	#, D.37163
	movl	%ecx, %eax	# c, c
#APP
# 177 "./arch/x86/include/asm/atomic.h" 1
	.pushsection .smp_locks,"a"
.balign 4
.long 671f - .
.popsection
671:
	lock; cmpxchgl %edx,ghes_in_nmi(%rip)	# D.37163, MEM[(volatile u32 *)&ghes_in_nmi]
# 0 "" 2
#NO_APP

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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