linux-kernel - Re: x86: Inconsistent xAPIC synchronization in arch_irq_work

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Message-ID: <20140121140113.GL30183@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jan 2014 15:01:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86: Inconsistent xAPIC synchronization in arch_irq_work_raise?

On Tue, Jan 21, 2014 at 02:02:06PM +0100, Jan Kiszka wrote:
> 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.

Could be I had no clue what I was doing and copy/pasted the code until
it compiled and ran.

In fact, I've got no clue what an ICR is.
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