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Date:	Sun, 9 Mar 2014 08:57:19 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Davidlohr Bueso <davidlohr@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Davidlohr Bueso <davi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] mm: per-thread vma caching

On Sun, Mar 9, 2014 at 5:57 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> No, dup_task_struct() is obviously lockless. And the new child is not yet
> visible to for_each_process_thread().

Ok, then the siimple approach is to just do

    /* Did we miss an invalidate event? *
    if (mm->seqcount < tsk->seqcount)
        clear_vma_cache();

after making the new thread visible.

Then the "race" becomes one of "we cannot have 4 billion mmap/munmap
events in other threads while we're setting up a new thread", which I
think is fine.

               Linus
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