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

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Date:	Mon, 27 Apr 2015 10:46:31 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"Zheng, Lv" <lv.zheng@...el.com>
Cc:	linux-edac <linux-edac@...r.kernel.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>,
	"Chen, Gong" <gong.chen@...ux.intel.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 5/5] GHES: Make NMI handler have a single reader

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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