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

[Cliff Wickman <cpw@sgi.com>]

Hi Ingo,

On Wed, Jun 15, 2011 at 05:54:45PM +0200, Ingo Molnar wrote:
> 
> * Cliff Wickman <cpw@sgi.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@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.
> 
> 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.

You're right of course. But we don't support CPU hotplug on the UV hardware.
There are enhancements needed in both the BIOS and Linux (BAU and GRU among
them). They are on our work queue.

-Cliff
-- 
Cliff Wickman
SGI
cpw@sgi.com
(651) 683-3824