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Date: Thu, 2 Apr 2015 13:57:33 -0700
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Subject: Re: smp_call_function_single lockups
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Chris J Arges <chris.j.arges@canonical.com>,
        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>,
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On Thu, Apr 2, 2015 at 12:07 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> So one possibility would be that an 'IPI was sent but lost'.

Yes, the "sent but lost" thing would certainly explain the lockups.

At the same time, that sounds like a huge hardware bug, and that's
somewhat surprising/unlikely.

That said.

> We could try the following trick: poll for completion for a couple of
> seconds (since an IPI is not held up by anything but irqs-off
> sections, it should arrive within microseconds typically - seconds of
> polling should be more than enough), and if the IPI does not arrive,
> print a warning message and re-send the IPI.

Sounds like a reasonable approach. At worst it doesn't fix anything,
and we never see any messages, and that tells us something too.

                        Linus