x86: don't send an IPI to the empty set of CPU's

The default_send_IPI_mask_logical() function uses the "flat" APIC mode
to send an IPI to a set of CPU's at once, but if that set happens to be
empty, some older local APIC's will apparently be rather unhappy.  So
just warn if a caller gives us an empty mask, and ignore it.

This fixes a regression in 2.6.30.x, due to commit 4595f962 ("x86:
change flush_tlb_others to take a const struct cpumask"), documented
here:

	http://bugzilla.kernel.org/show_bug.cgi?id=13933

which causes a silent lock-up.  It only seems to happen on PPro, P2, P3
and Athlon XP cores.  Most developers sadly (or not so sadly, if you're
a developer..) have more modern CPU's.  Also, on x86-64 we don't use the
flat APIC mode, so it would never trigger there even if the APIC didn't
like sending an empty IPI mask.
Reported-by: Pavel Vilim <wylda@volny.cz>
Reported-and-tested-by: Thomas Björnell <thomas.bjornell@gmail.com>
Reported-and-tested-by: Martin Rogge <marogge@onlinehome.de>
Cc: Mike Travis <travis@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/apic/ipi.c

@@ -106,6 +106,9 @@ void default_send_IPI_mask_logical(const struct cpumask *cpumask, int vector)
 	unsigned long mask = cpumask_bits(cpumask)[0];
 	unsigned long flags;
 
+	if (WARN_ONCE(!mask, "empty IPI mask"))
+		return;
+
 	local_irq_save(flags);
 	WARN_ON(mask & ~cpumask_bits(cpu_online_mask)[0]);
 	__default_send_IPI_dest_field(mask, vector, apic->dest_logical);