linux-kernel - x86: Inconsistent xAPIC synchronization in arch_irq_work

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Date:	Tue, 21 Jan 2014 14:02:06 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: x86: Inconsistent xAPIC synchronization in arch_irq_work_raise?

Hi all,

while trying to plug a race in the CPU hotplug code on xAPIC systems, I
was analyzing IPI transmission patterns. The handlers in
arch/x86/include/asm/ipi.h first wait for ICR, then send. In contrast,
arch_irq_work_raise sends the self-IPI directly and then waits. This
looks inconsistent. Is it intended?

BTW, the races are in wakeup_secondary_cpu_via_init and
wakeup_secondary_cpu_via_nmi (lacking IRQ disable around ICR accesses).
There we also send first, then wait for completion. But I guess that is
due to the code originally only being used during boot. Will send fixes
for those once the sync pattern is clear to me.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
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