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    Subject[PATCH] workaround minor lockdep bug triggered by mm_take_all_locks
    From: Andrea Arcangeli <andrea@qumranet.com>

    Lockdep can't recognize if spinlocks are at a different address. So
    using trylock in a loop is one way to avoid lockdep to generate false
    positives. After lockdep will be fixed this change can and should be
    reverted.

    Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
    ---

    Resubmit because I didn't update the subject and so I improved the
    comments too. This is mostly to show it's a bug in lockdep and it can
    be trivially worked around without having to fix lockdep for real, any
    superior solution to this hack is more than welcome and recommended.

    diff -r 3469dce61df1 mm/mmap.c
    --- a/mm/mmap.c Tue Jul 29 20:01:28 2008 +0200
    +++ b/mm/mmap.c Mon Aug 04 19:54:27 2008 +0200
    @@ -2279,8 +2279,12 @@ static void vm_lock_anon_vma(struct anon
    /*
    * The LSB of head.next can't change from under us
    * because we hold the mm_all_locks_mutex.
    + *
    + * spin_lock would confuse lockdep who doesn't notice
    + * the 'anon_vma' always changing address.
    */
    - spin_lock(&anon_vma->lock);
    + while (!spin_trylock(&anon_vma->lock))
    + cpu_relax();
    /*
    * We can safely modify head.next after taking the
    * anon_vma->lock. If some other vma in this mm shares
    @@ -2310,7 +2314,12 @@ static void vm_lock_mapping(struct addre
    */
    if (test_and_set_bit(AS_MM_ALL_LOCKS, &mapping->flags))
    BUG();
    - spin_lock(&mapping->i_mmap_lock);
    + /*
    + * spin_lock would confuse lockdep who doesn't notice
    + * the 'mapping' always changing address.
    + */
    + while (!spin_trylock(&mapping->i_mmap_lock))
    + cpu_relax();
    }
    }


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