linux-kernel - Re: [PATCH] x86, hugetlb: add missing TLB page invalidation for hugetlb

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Date:	Wed, 14 May 2014 10:24:34 -0700
From:	Dave Hansen <dave@...1.net>
To:	Anthony Iliopoulos <anthony.iliopoulos@...wei.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Shay Goikhman <shay.goikhman@...wei.com>,
	Paul Mundt <paul.mundt@...wei.com>,
	Carlos Villavieja <villavieja@....utexas.edu>,
	Nacho Navarro <nacho.navarro@....es>,
	Avi Mendelson <avi.mendelson@....technion.ac.il>,
	Yoav Etsion <yetsion@....technion.ac.il>,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	David Gibson <david@...son.dropbear.id.au>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] x86, hugetlb: add missing TLB page invalidation for hugetlb_cow()

On 05/15/2014 10:00 AM, Anthony Iliopoulos wrote:
> I have actually also wondered about another related thing:
> even when we actually do invalidate the page, there may still be a
> race between a thread on a core that reads the page in some tight
> loop (e.g. on a spinlock), and the page fault handler running on
> a different core, at the point where the pte is set. Since we
> invalidate the page via the TLB shootdowns *before* we update
> the pte (this is true for all do_wp_page(), do_huge_pmd_wp_page()
> as well as hugetlb_cow()), there may be some tiny window where the
> thread might re-read the page before the pte is set.

Don't forget about the "clear" part.  ptep_clear_flush() does:

        pte = ptep_get_and_clear(mm, address, ptep);
        if (pte_accessible(mm, pte))
                flush_tlb_page(vma, address);

so it makes the pte !present and guarantees that any other CPUs looking
at it after the flush but before the set_pte() will also end up in the
page fault handler, and they'll wait until the first fault has finished
with the page tables.
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