Messages in this thread | | | Date | Wed, 21 Dec 2005 07:20:25 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 04/15] Generic Mutex Subsystem, add-atomic-call-func-x86_64.patch |
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* Nick Piggin <nickpiggin@yahoo.com.au> 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.
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