Re: [PATCH 1 of 6] x86, UV: smp_processor_id in a preemptable region

[Cliff Wickman <cpw@sgi.com>]

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@sgi.com> wrote:
> > From: Cliff Wickman <cpw@sgi.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@sgi.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.
It is possible that the found per-cpu structure could differ from cpu to
cpu. But if this tunable is thus caused to be set too high, a 'throttling'
upper bound will not be enforced. No other harm is done.
But yes, I should have noted that an ugly DEBUG_PREEMPT warning will
be the worst effect.
-Cliff

> 
> > ? ? ? ?bcp = &per_cpu(bau_control, smp_processor_id());
> > + ? ? ? preempt_enable_no_resched();
> >
> > ? ? ? ?ret = parse_tunables_write(bcp, instr, count);
> > ? ? ? ?if (ret)
> > --
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-- 
Cliff Wickman
SGI
cpw@sgi.com
(651) 683-3824