RE: [PATCH 1/4] x86/mce: do not overwrite no_way_out if mce_end() fails

From: "Paoloni, Gabriele" <gabriele.paoloni@intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: "Luck, Tony" <tony.luck@intel.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-safety@lists.elisa.tech" <linux-safety@lists.elisa.tech>
Subject: RE: [PATCH 1/4] x86/mce: do not overwrite no_way_out if mce_end() fails
Date: Fri, 20 Nov 2020 17:31:32 +0000	[thread overview]
Message-ID: <MN2PR11MB4158BE8B738CF6F6BF9B850D88FF0@MN2PR11MB4158.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20201120170737.GD712@zn.tnic>

Hi Boris

> -----Original Message-----
> From: Borislav Petkov <bp@alien8.de>
> Sent: Friday, November 20, 2020 6:08 PM
> To: Paoloni, Gabriele <gabriele.paoloni@intel.com>
> Cc: Luck, Tony <tony.luck@intel.com>; tglx@linutronix.de;
> mingo@redhat.com; x86@kernel.org; hpa@zytor.com; linux-
> edac@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
> safety@lists.elisa.tech
> Subject: Re: [PATCH 1/4] x86/mce: do not overwrite no_way_out if
> mce_end() fails
> 
> On Wed, Nov 18, 2020 at 03:15:49PM +0000, Gabriele Paoloni wrote:
> > Currently if mce_end() fails no_way_out is set equal to worst.
> > worst is the worst severirty that was found in the MCA banks
> 		     ^^^^^^^^^
> 
> Please introduce a spellchecker into your patch creation workflow.
> 
> > associated to the current CPU; however at this point no_way_out
> 	     ^
> 	     with
> 
> 
> > could be already set by mca_start() by looking at all severities
> 
> I think you mean "could have been already set" here
> 
> > of all CPUs that entered the MCE handler.
> > if mce_end() fails we first check if no_way_out is already set and
> 
> Please use passive voice in your commit message: no "we" or "I", etc.
> 
> Also, pls start new sentences with a capital letter and end them with a
> fullstop.

Sorry about the grammar errors above, I'll pay more attention in future

> 
> > if so we stick to it, otherwise we use the local worst value
> 
> So basically you're trying to say here that no_way_out might have been
> already set and other CPUs could overwrite it and that should not
> happen.
> 
> Is that what you mean?

I mean that on this CPU thread at this point mce_start() already cached
global_nwo and hence could accumulate fatal severities of other CPUs.

Now here if mce_end() fails we only consider the local 'worst' severity
and we overwrite those already cached.

> 
> > Signed-off-by: Gabriele Paoloni <gabriele.paoloni@intel.com>
> > Reviewed-by: Tony Luck <tony.luck@intel.com>
> > ---
> >  arch/x86/kernel/cpu/mce/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/cpu/mce/core.c
> b/arch/x86/kernel/cpu/mce/core.c
> > index 4102b866e7c0..b990892c6766 100644
> > --- a/arch/x86/kernel/cpu/mce/core.c
> > +++ b/arch/x86/kernel/cpu/mce/core.c
> > @@ -1385,7 +1385,7 @@ noinstr void do_machine_check(struct pt_regs
> *regs)
> >  	 */
> >  	if (!lmce) {
> >  		if (mce_end(order) < 0)
> > -			no_way_out = worst >= MCE_PANIC_SEVERITY;
> > +			no_way_out = no_way_out ? no_way_out : worst >=
> MCE_PANIC_SEVERITY;
> 
> I had to stare at this a bit to figure out what you're doing. So how
> about simplifying this:
> 
> 			if (!no_way_out)
> 				no_way_out = worst >=

Yes that works as well improving readability.

If ok I will fix the grammar and rewrite this code in v2.

Many Thanks
Gab

> MCE_PANIC_SEVERITY;
> 
> ?
> 
> Thx.
> 
> --
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette
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