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    Subject[patch 15/19] x86: fix global_flush_tlb() bug
    -stable review patch.  If anyone has any objections, please let us know.

    From: Ingo Molnar <>

    patch 9a24d04a3c26c223f22493492c5c9085b8773d4a upstream

    While we were reviewing pageattr_32/64.c for unification,
    Thomas Gleixner noticed the following serious SMP bug in

    list_replace_init(&deferred_pages, &l);

    this is SMP-unsafe because list_replace_init() done on two CPUs in
    parallel can corrupt the list.

    This bug has been introduced about a year ago in the 64-bit tree:

    commit ea7322decb974a4a3e804f96a0201e893ff88ce3
    Author: Andi Kleen <>
    Date: Thu Dec 7 02:14:05 2006 +0100

    [PATCH] x86-64: Speed and clean up cache flushing in change_page_attr

    - dpage = xchg(&deferred_pages, NULL);
    + list_replace_init(&deferred_pages, &l);

    the xchg() based version was SMP-safe, but list_replace_init() is not.
    So this "cleanup" introduced a nasty bug.

    why this bug never become prominent is a mystery - it can probably be
    explained with the (still) relative obscurity of the x86_64 architecture.

    the safe fix for now is to write-lock init_mm.mmap_sem.

    Signed-off-by: Ingo Molnar <>
    Signed-off-by: Thomas Gleixner <>
    Cc: Andi Kleen <>
    Cc: Andrew Morton <>
    Signed-off-by: Greg Kroah-Hartman <>

    arch/x86_64/mm/pageattr.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- a/arch/x86_64/mm/pageattr.c
    +++ b/arch/x86_64/mm/pageattr.c
    @@ -229,9 +229,14 @@ void global_flush_tlb(void)
    struct page *pg, *next;
    struct list_head l;

    - down_read(&init_mm.mmap_sem);
    + /*
    + * Write-protect the semaphore, to exclude two contexts
    + * doing a list_replace_init() call in parallel and to
    + * exclude new additions to the deferred_pages list:
    + */
    + down_write(&init_mm.mmap_sem);
    list_replace_init(&deferred_pages, &l);
    - up_read(&init_mm.mmap_sem);
    + up_write(&init_mm.mmap_sem);


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