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    SubjectRe: 2.6.6-rc3-mm1
    Date
    On Fri, 30 Apr 2004 01:46:58 -0700, 
    Andrew Morton <akpm@osdl.org> wrote:
    >ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.6-rc3/2.6.6-rc3-mm1/
    >+allow-architectures-to-reenable-interrupts-on-contended-spinlocks.patch
    >
    > Rework the spinlock code so that architectures can reenable interrupts when
    > spinning in spin_lock_irq() or spin_lcok_irqsave(). Only implemented for
    > ia64 at this stage.

    Only spin_lock_irqsave(), not spin_lock_irq(). The patch needs the old
    flags to determine if interrupts were originally enabled, the flags are
    not saved for spin_lock_irq().

    In theory, spin_lock_irq() should never be called when interrupts are
    already disabled, the corresponding spin_unlock_irq() will
    unconditionally enable interrupts. So it should be possible for
    spin_lock_irq() to pass a constant flags value to
    _raw_spin_lock_flags(), stating that interrupts were enabled before
    spin_lock_irq(). Two problems with that :-

    * The flags value for 'interrupts were enabled' is arch specific.

    * I have seen buggy code that does spin_lock_irq() when interrupts
    are already disabled. Unconditionally enabling interrupts while
    waiting for a contended spin_lock_irq() will perturb that code.

    For now, the patch only improves the performance of spin_lock_irqsave().

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