linux-kernel - Re: [PATCH 08/10] NOTIFIER: Take over TIF_MCE_NOTIFY and implement task return notifier

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Date:	Sun, 12 Jun 2011 22:31:58 -0700
From:	Tony Luck <tony.luck@...el.com>
To:	Borislav Petkov <bp@...64.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Huang, Ying" <ying.huang@...el.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH 08/10] NOTIFIER: Take over TIF_MCE_NOTIFY and implement
 task return notifier

On Sun, Jun 12, 2011 at 3:38 PM, Borislav Petkov <bp@...64.org> wrote:
> On Thu, Jun 09, 2011 at 05:36:42PM -0400, Luck, Tony wrote:
>> From: Tony Luck <tony.luck@...el.com>
>>
>> Existing user return notifier mechanism is designed to catch a specific
>> cpu just as it returns to run any task in user mode.  We also need a
>> mechanism to catch a specific task.
>
> Why do we need that? I mean, in the remaining patches we end up either
> running memory_failure() or sending signals to a task. Can't we do it
> all in the user return notifier and not have a different notifier for
> each policy?

Unless I'm mis-reading the user-return-notifier code, it is possible that
we'll context switch before we get to the notifier. At that point the
user-return-notifier TIF bit is passed on from our task to the newly
run-able task. But our task is still viable, so another cpu could grab
it and start running it ... then we have a race ... will the new task
that inherited the notifier unmap the page fast enough, or will there
be a loud BANG as the original task runs right into the machine
check again.

-Tony
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