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

[Zheng, Lv]

Hi,

> From: Borislav Petkov [mailto:bp@xxxxxxxxx]
> Sent: Monday, April 27, 2015 4:47 PM
> 
> 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?

My mistake.
I mean cmpxchg() and xchg() (or atomic_cmpxchg() and atomic_xchg()) pair here, so nothing can be reduced.

But IMO, atomic_add_unless() is implemented via cmpxchg on many architectures.
And it might be better to use it directly here which is a bit faster as you actually only need one value switch here.

Thanks and best regards
-Lv


> 
> --
> Regards/Gruss,
>     Boris.
> 
> ECO tip #101: Trim your mails when you reply.
> --