x86/hyperv: check cpu mask after interrupt has been disabled

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
Link: https://lore.kernel.org/r/20210105175043.28325-1-wei.liu@kernel.org
Reviewed-by:  Michael Kelley <mikelley@microsoft.com>

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);