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

Borislav Petkov
2015-04-28T13:59:13Z

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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