* Nick Piggin <[email protected]> wrote:
> >>Considering that on UP, the arm should not need to disable interrupts
> >>for this function (or has someone refuted Linus?), how about:
> >
> >Kernel preemption.
>
> preempt_disable() ?
please take a look at kernel/mutex.c, there's a define at the top of the
file:
// #define MUTEX_IRQ_SAFE
which, if off, makes the mutex code use preempt_disable() and
preempt_enable() to make it preemption-safe. If it's on, the mutex
implementation uses IRQ flags.
in my current tree i've already eliminated this define, and have
switched the code to use preempt_disable()/preempt_enable() exclusively,
because preempt_*() is equally fast on all platforms, while IRQ disable
costs vary largely. (and they are rarely faster than preempt_disable()).
my current tree also provides a mechanism for architectures to hand-code
the mutex lock and unlock fastpath, if they choose to do so. So i think
we can really stop the cycle counting.
Ingo
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