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    SubjectRe: [PATCH] writeback: avoid race when update bandwidth
    On Tue, Jun 12, 2012 at 07:21:29PM +0800, Fengguang Wu wrote:
    > On Tue, Jun 12, 2012 at 06:26:43PM +0800, Wanpeng Li wrote:
    > > From: Wanpeng Li <>
    > That email address is no longer in use?
    > > Since bdi->wb.list_lock is used to protect the b_* lists,
    > > so the flushers who call wb_writeback to writeback pages will
    > > stuck when bandwidth update policy holds this lock. In order
    > > to avoid this race we can introduce a new bandwidth_lock who
    > > is responsible for protecting bandwidth update policy.

    This is not a race condition - it is a lock contention condition.

    > This looks good to me. wb.list_lock could be contended and it's better
    > for bdi_update_bandwidth() to use a standalone and hardly contended
    > lock.

    I'm not sure it will be "hardly contended". That's a global lock, so
    now we'll end up with updates on different bdis contending and it's
    not uncommon to see a couple of thousand processes on large machines
    beating on balance_dirty_pages(). Putting a global scope lock
    around such a function doesn't seem like a good solution to me.

    Oh, and if you want to remove the dirty_lock from
    global_update_limit(), then replacing the lock with a cmpxchg loop
    will do it just fine....


    Dave Chinner

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