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    SubjectRe: [ltt-dev] [PATCH] Fix dirty page accounting in redirty_page_for_writepage()
    On Thu, 30 Apr 2009, Mathieu Desnoyers wrote:

    > > The 3 variants on x86 generate the same instructions. On other platforms
    > > they would need to be able to fallback in various way depending on the
    > > availability of instructions that are atomic vs. preempt or irqs.
    > >
    > The problem here, as we did figure out a while ago with the atomic
    > slub we worked on a while ago, is that if we have the following code :
    > local_irq_save
    > var++
    > var++
    > local_irq_restore
    > that we would like to turn into irq-safe percpu variant with this
    > semantic :
    > percpu_add_irqsafe(var)
    > percpu_add_irqsafe(var)
    > We are generating two irq save/restore in the fallback, which will be
    > slow.
    > However, we could do the following trick :
    > percpu_irqsave(flags);
    > percpu_add_irq(var);
    > percpu_add_irq(var);
    > percpu_irqrestore(flags);

    Hmmm.I do not remember any of those double ops in the patches that I did a
    while back for this. It does not make sense either because atomic per cpu
    ops are only atomic for a single instruction. You are trying to extend
    that so that multiple "atomic" instructions are now atomic.

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      Last update: 2009-04-30 16:25    [W:0.031 / U:26.972 seconds]
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