linux-kernel - Re: [PATCH 1 of 6] x86, UV: smp_processor

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Date:	Wed, 15 Jun 2011 17:54:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cliff Wickman <cpw@....com>
Cc:	Rakib Mullick <rakib.mullick@...il.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1 of 6] x86, UV: smp_processor_id in a preemptable region


* Cliff Wickman <cpw@....com> wrote:

> On Wed, Jun 15, 2011 at 12:05:17PM +0600, Rakib Mullick wrote:
> > On Wed, Jun 15, 2011 at 5:06 AM, Cliff Wickman <cpw@....com> wrote:
> > > From: Cliff Wickman <cpw@....com>
> > >
> > > Calling smp_processor_id() from within a preemptable region will issue
> > > a warning if DEBUG_PREEMPT is set.
> > >
> > > Diffed against 3.0.0-rc3
> > >
> > > Signed-off-by: Cliff Wickman <cpw@....com>
> > > ---
> > > ?arch/x86/platform/uv/tlb_uv.c | ? ?2 ++
> > > ?1 file changed, 2 insertions(+)
> > >
> > > Index: linux/arch/x86/platform/uv/tlb_uv.c
> > > ===================================================================
> > > --- linux.orig/arch/x86/platform/uv/tlb_uv.c
> > > +++ linux/arch/x86/platform/uv/tlb_uv.c
> > > @@ -1334,7 +1334,9 @@ static ssize_t tunables_write(struct fil
> > >
> > > ? ? ? ?instr[count] = '\0';
> > >
> > > + ? ? ? preempt_disable(); /* avoid DEBUG_PREEMPT warning */
> > 
> > I think above code comment, "avoid DEBUG_PREEMPT warning" should be to
> > something more meaningful. It's a BUG, if smp_processor_id() is called
> > within preemptible context. So, we don't want to hit that BUG.
> 
> I agree that calling smp_processor_id() within a preemptible context is
> going to produce unpredictable results.  In this particular case we just
> need a valid cpu number so that we can find a per-cpu structure.
> That structure contains a reasonable (sanity-checking) limit to the value
> of the tunable that is being written.

So what happens if the code gets preempted away and this CPU is 
hotplugged away? You'll reference a CPU ID that does not exist 
anymore.

Thanks,

	Ingo
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