linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Date:	Mon, 28 Apr 2008 11:13:49 -0400
From:	Mark Lord <lkml@....ca>
To:	Pavel Machek <pavel@....cz>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, npiggin@...e.de,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

Pavel Machek wrote:
> Hi!
> 
>>>>> The second bug, is that for the halt case at least,
>>>>> nobody waits for the other CPU to actually halt
>>>>> before continuing.. so we sometimes enter the shutdown
>>>>> code while other CPUs are still active.
>>>>>
>>>>> This causes some machines to hang at shutdown,
>>>>> unless CPU_HOTPLUG is configured and takes them offline
>>>>> before we get here.
>>>> I'm guessing there's a reason it doesn't pass '1' as the last argument,
>>>> because that would fix that issue?
>>> Undoubtedly -- perhaps the called CPU halts, and therefore cannot reply.  :)
>> Uhm yes, I guess stop_this_cpu() does exactly what the name implies :-)
>>
>>> But some kind of pre-halt ack, perhaps plus a short delay by the caller
>>> after receipt of the ack, would probably suffice to kill that bug.
>>>
>>> But I really haven't studied this code enough to know,
>>> other than that it historically has been a sticky area
>>> to poke around in.
>> Something like this will close the window to right up until the point
>> where the other CPUs have 'almost' called halt().
> 
> Now I took a look at context... why not simply use same trick swsusp
> uses, and do a hot unplug of all cpus at the end of shutdown?
..

That's the current existing workaround for this bug,
but not everybody has cpu hotplug in their config,
and this bug should still get fixed.

Cheers
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