Messages in this thread |  | | Date | Wed, 4 Sep 2002 15:21:35 -0700 | From | Bob Miller <> | Subject | Re: Questions on semaphores |
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On Wed, Sep 04, 2002 at 11:29:31PM +0200, Juan M. de la Torre wrote: > > Hi people, I have two question regarding the i386 semaphore implementation > in kernel 2.4.19. > > Please dont blame me if they are too obvius; i'm a newbie in kernel hacking > :) > > The functions __down, __down_interruptible and __down_trylock (defined > in arch/i386/kernel/semaphore.c) use the global spinlock > 'semaphore_lock' to access some fields of the semaphore they are > working on: > > 1) Is there any reason to do this?
It was easy to do.
> 2) Wouldn't it be more scalable to use a per-semaphore lock instead a > global spinlock? >
Yes it would be more scalable, but not as much as you would think. The __down, __down_interruptible and __down_trylock code only gets invoked when the semaphore is contended for.
> The function __down_trylock try to get the spinlock using > spin_lock_irqsave, instead of using spin_lock_irq: > > 1) why? :) >
The __down_trylock() code can be called with another lock held. The spin_lock_irqsave()/spin_lock_irqrestore() interface is used to restore the irq value for the lock that may already be held.
> Thanks in advance, > Juanma > The code in the 2.5 tree was changed a while back to use the spinlock in the wait_queue_head_t to replace the global semaphore spin lock. So, this has been "FIXED" in 2.5.
-- Bob Miller Email: rem@osdl.org Open Source Development Lab Phone: 503.626.2455 Ext. 17 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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