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Date: Fri, 20 Feb 2015 12:03:03 -0800
Message-ID: <CA+55aFxcVNyogK6wc8B6ZjSZk1czwF9XRPc69C_Y52+hUQYSCA@mail.gmail.com>
Subject: Re: smp_call_function_single lockups
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Rafael David Tinoco <inaddy@ubuntu.com>, Peter Anvin <hpa@zytor.com>,
        Jiang Liu <jiang.liu@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Gema Gomez <gema.gomez-solano@canonical.com>,
        Christopher Arges <chris.j.arges@canonical.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Fri, Feb 20, 2015 at 11:41 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> I'm not so sure about that aspect: I think disabling IRQs
> might be necessary with some APICs (if lower levels don't
> disable IRQs), to make sure the 'local APIC busy' bit isn't
> set:

Right. But afaik not for the x2apic case, which this is.  The x2apic
doesn't even have a busy bit, and sending the ipi is a single write,

I agree that when doing other apic implementations, we may need to
guarantee atomicity for things like "wait for apic idle, then send the
ipi".

                        Linus
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