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

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Date:	Tue, 21 Jan 2014 15:11:21 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 2014-01-21 15:01, 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.

Old xAPIC requires you to only send IPIs, when the APIC signals it is
done with sending the previous one. Therefore we wait for availability
in the other IPI transmission services before writing to ICR.

OK, then I will write a separate patch for arch_irq_work_raise to switch
the ordering.

Jan

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