Messages in this thread | | | Date | Tue, 28 Apr 2015 15:59:13 +0200 | From | Borislav Petkov <> | Subject | Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader |
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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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