Re: [PATCH] x86/hyperv: check cpu mask after interrupt has been disabled

[Wei Liu <wei.liu@kernel.org>]

On Tue, Jan 05, 2021 at 06:20:05PM +0000, Michael Kelley wrote:
> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, January 5, 2021 9:51 AM
> > 
> > We've observed crashes due to an empty cpu mask in
> > hyperv_flush_tlb_others.  Obviously the cpu mask in question is changed
> > between the cpumask_empty call at the beginning of the function and when
> > it is actually used later.
> > 
> > One theory is that an interrupt comes in between and a code path ends up
> > changing the mask. Move the check after interrupt has been disabled to
> > see if it fixes the issue.
> > 
> > Signed-off-by: Wei Liu <wei.liu@kernel.org>
> > Cc: stable@kernel.org
> > ---
> >  arch/x86/hyperv/mmu.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
> > index 5208ba49c89a..2c87350c1fb0 100644
> > --- a/arch/x86/hyperv/mmu.c
> > +++ b/arch/x86/hyperv/mmu.c
> > @@ -66,11 +66,17 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
> >  	if (!hv_hypercall_pg)
> >  		goto do_native;
> > 
> > -	if (cpumask_empty(cpus))
> > -		return;
> > -
> >  	local_irq_save(flags);
> > 
> > +	/*
> > +	 * Only check the mask _after_ interrupt has been disabled to avoid the
> > +	 * mask changing under our feet.
> > +	 */
> > +	if (cpumask_empty(cpus)) {
> > +		local_irq_restore(flags);
> > +		return;
> > +	}
> > +
> >  	flush_pcpu = (struct hv_tlb_flush **)
> >  		     this_cpu_ptr(hyperv_pcpu_input_arg);
> > 
> > --
> > 2.20.1
> 
> Reviewed-by:  Michael Kelley <mikelley@microsoft.com>
> 

Applied to hyperv-fixes.

Wei.