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SubjectRe: x86: Inconsistent xAPIC synchronization in arch_irq_work_raise?
On 2014-01-21 15:11, Jan Kiszka wrote:
> 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.

Hmm, missed that we do have synchronization already via
apic->send_IPI_self -> default_send_IPI_self ->
__default_send_IPI_shortcut. So the closing wait would only be relevant
if we need to settle the APIC because we may have interrupted a
wait_icr_idle + write_icr sequence. But shouldnt those sequences be
atomic (except for the problematic wakeup_secondary path)?

Still confused: What is the official locking model around wait_icr_idle,
write(ICR), and also write(IRC2)? IRQ disable around ICR2+ICR accesses
and preempt_disable around wait + write? That is also important to fix
the SMP boot-up code properly.

Jan

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


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