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

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Message-ID: <20140121141058.GA4559@gmail.com>
Date:	Tue, 21 Jan 2014 15:10:58 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>, 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?


* Peter Zijlstra <peterz@...radead.org> 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.

APIC ICR = Interrupt Command Register - this is the MMIO-mapped 
register of the local APIC that (when written to) triggers the sending 
of IPIs and (when read from) shows the status how the IPI is going or 
whether a new IPI can be sent.

( Not to be confused with the APIC timer ICR, which is 'Initial Count
  Register' and does something entirely different. )

Thanks,

	Ingo
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