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Subject[patch 04/71] x86: dont send an IPI to the empty set of CPUs
2.6.30-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Linus Torvalds <torvalds@linux-foundation.org>

commit 83d349f35e1ae72268c5104dbf9ab2ae635425d4 upstream.

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 4595f9620 ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
arch/x86/kernel/apic/ipi.c | 3 +++
1 file changed, 3 insertions(+)

--- a/arch/x86/kernel/apic/ipi.c
+++ b/arch/x86/kernel/apic/ipi.c
@@ -106,6 +106,9 @@ void default_send_IPI_mask_logical(const
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);

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