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From: Andy Lutomirski <luto@amacapital.net>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>, Borislav Petkov <bp@suse.de>,
	Andy Lutomirski <luto@kernel.org>, Rik van Riel <riel@redhat.com>,
	Mel Gorman <mgorman@suse.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/mm: disable preemption during CR3 read+write
Date: Fri, 5 Aug 2016 08:42:32 -0700	[thread overview]
Message-ID: <CALCETrV9n=-Zi2KBT7i-WLrYGffXy1ha+M=_PhvnuOiG7pim8A@mail.gmail.com> (raw)
In-Reply-To: <1470404259-26290-1-git-send-email-bigeasy@linutronix.de>

On Fri, Aug 5, 2016 at 6:37 AM, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> Usually current->mm (and therefore mm->pgd) stays the same during the
> lifetime of a task so it does not matter if a task gets preempted during
> the read and write of the CR3.
>
> But then, there is this scenario on x86-UP:
> TaskA is in do_exit() and exit_mm() sets current->mm = NULL followed by
> mmput() -> exit_mmap() -> tlb_finish_mmu() -> tlb_flush_mmu() ->
> tlb_flush_mmu_tlbonly() -> tlb_flush() -> flush_tlb_mm_range() ->
> __flush_tlb_up() -> __flush_tlb() ->  __native_flush_tlb().
>
> At this point current->mm is NULL but current->active_mm still points to
> the "old" mm.
> Let's preempt taskA _after_ native_read_cr3() by taskB. TaskB has its
> own mm so CR3 has changed.
> Now preempt back to taskA. TaskA has no ->mm set so it borrows taskB's
> mm and so CR3 remains unchanged. Once taskA gets active it continues
> where it was interrupted and that means it writes its old CR3 value
> back. Everything is fine because userland won't need its memory
> anymore.

This should affect kernel threads too, right?

Acked-by: Andy Lutomirski <luto@kernel.org>

WARNING: multiple messages have this Message-ID (diff)
From: Andy Lutomirski <luto@amacapital.net>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>, Borislav Petkov <bp@suse.de>,
	Andy Lutomirski <luto@kernel.org>, Rik van Riel <riel@redhat.com>,
	Mel Gorman <mgorman@suse.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/mm: disable preemption during CR3 read+write
Date: Fri, 5 Aug 2016 08:42:32 -0700	[thread overview]
Message-ID: <CALCETrV9n=-Zi2KBT7i-WLrYGffXy1ha+M=_PhvnuOiG7pim8A@mail.gmail.com> (raw)
In-Reply-To: <1470404259-26290-1-git-send-email-bigeasy@linutronix.de>

On Fri, Aug 5, 2016 at 6:37 AM, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> Usually current->mm (and therefore mm->pgd) stays the same during the
> lifetime of a task so it does not matter if a task gets preempted during
> the read and write of the CR3.
>
> But then, there is this scenario on x86-UP:
> TaskA is in do_exit() and exit_mm() sets current->mm = NULL followed by
> mmput() -> exit_mmap() -> tlb_finish_mmu() -> tlb_flush_mmu() ->
> tlb_flush_mmu_tlbonly() -> tlb_flush() -> flush_tlb_mm_range() ->
> __flush_tlb_up() -> __flush_tlb() ->  __native_flush_tlb().
>
> At this point current->mm is NULL but current->active_mm still points to
> the "old" mm.
> Let's preempt taskA _after_ native_read_cr3() by taskB. TaskB has its
> own mm so CR3 has changed.
> Now preempt back to taskA. TaskA has no ->mm set so it borrows taskB's
> mm and so CR3 remains unchanged. Once taskA gets active it continues
> where it was interrupted and that means it writes its old CR3 value
> back. Everything is fine because userland won't need its memory
> anymore.

This should affect kernel threads too, right?

Acked-by: Andy Lutomirski <luto@kernel.org>

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  parent reply	other threads:[~2016-08-05 15:42 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-08-05 13:37 [PATCH] x86/mm: disable preemption during CR3 read+write Sebastian Andrzej Siewior
2016-08-05 13:37 ` Sebastian Andrzej Siewior
2016-08-05 13:53 ` Peter Zijlstra
2016-08-05 13:53   ` Peter Zijlstra
2016-08-05 14:38 ` Rik van Riel
2016-08-05 15:42 ` Andy Lutomirski [this message]
2016-08-05 15:42   ` Andy Lutomirski
2016-08-05 15:52   ` Sebastian Andrzej Siewior
2016-08-05 15:52     ` Sebastian Andrzej Siewior
2016-08-10 18:10 ` [tip:x86/urgent] x86/mm: Disable " tip-bot for Sebastian Andrzej Siewior
2017-10-17 15:57 [PATCH] " Sebastian Andrzej Siewior
2017-10-18 15:01 ` Bernhard Kaindl
2017-10-18 16:51   ` Greg KH

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