[PATCH] x86: drop false warning of empty cpumask in IPI

[Gilad Ben-Yossef]

With current generic SMP infrastructure, it is feasible
for the target CPUs to begin processing an IPI work item
even before we sent them the actual IPI in the case that
an IPI from another CPU woke them first.

This can lead to generating a false warning in a valid
state of trying to send IPI with an empty cpumask when
multiple concurrent IPIs are being sent.

This patch was triggered by the following LKML discussion:
https://lkml.org/lkml/2012/1/13/308

Signed-off-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Milton Miller <miltonm@xxxxxxx>
CC: x86@xxxxxxxxxx
---
 arch/x86/kernel/apic/ipi.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/apic/ipi.c b/arch/x86/kernel/apic/ipi.c
index cce91bf..00b68a3 100644
--- a/arch/x86/kernel/apic/ipi.c
+++ b/arch/x86/kernel/apic/ipi.c
@@ -106,7 +106,10 @@ 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"))
+	if (!mask)
+		/* The target CPUs must have already processed the
+		 * work items due to a concurrent IPI
+		 */
 		return;
 
 	local_irq_save(flags);
-- 
1.7.0.4

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