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

Borislav Petkov
2015-04-27T08:46:49Z

On Mon, Apr 27, 2015 at 03:16:00AM +0000, Zheng, Lv wrote:
> > @@ -840,7 +840,9 @@ static int ghes_notify_nmi(unsigned int cmd, struct pt_regs *regs)
> > struct ghes *ghes;
> > int sev, ret = NMI_DONE;
> >
> > - raw_spin_lock(&ghes_nmi_lock);
> > + if (!atomic_add_unless(&ghes_in_nmi, 1, 1))
> > + return ret;
> > +
>
> Just a simple question.
> Why not just using cmpxchg here instead of atomic_add_unless so that no atomic_dec will be needed.

What do you think atomic_add_unless ends up doing:

#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.37056, MEM[(volatile u32 *)&ghes_in_nmi]
# 0 "" 2
#NO_APP

And you need to atomic_dec() so that another reader can enter, i.e. how
the exclusion primitive works.

Or did you have something else in mind?

--
Regards/Gruss,
Boris.

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