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

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Date:	Tue, 21 Jan 2014 15:51:05 +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>,
	Huang Ying <ying.huang@...el.com>
Subject: Re: x86: Inconsistent xAPIC synchronization in arch_irq_work_raise?

On Tue, Jan 21, 2014 at 03:01:13PM +0100, Peter Zijlstra wrote:
> 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.

I dug about a bit, I borrowed that code from:

lkml.kernel.org/r/1277348698-17311-3-git-send-email-ying.huang@...el.com

Huang Ying, can you explain to Jan why you do the wait afterwards?
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