Re: [Question] spin_lock in interrupt handler.

[liyu <liyu@ccoss.com.cn>]

Steven Rostedt wrote:

>On Fri, 2005-11-18 at 16:37 +0800, liyu wrote:
>  
>
>>Hi, every one in LKML.
>>
>>    I have one question about how to use spin_lock.
>>
>>    I read Documentation/spinlocks.txt wrote by Linus. The Lesson 1 and 
>>2 are simple for me.
>>But I confused in Lesson 3. The most doublt is why we can not use 
>>spin_lock_irq*() version in
>>interrupt handler?
>>
>>    At i386, I known the interrupt is disabled in interrupt handler. I 
>>think this feature is
>>supplied in handware-level. The spin_lock_irqrestore() will use  'sti'  
>>instruction internal, it will change interrupt mask bit in FLAGS 
>>register, do this have re-enable interrupt, even in interrput handler? I 
>>can not sure this.
>>    
>>
>
>Hello once again Liyu ;-)
>
>I don't see where he says you can't use spin_lock_irq* in interrupt
>handlers.  He only says that you are safe to use the non-irq* versions
>IFF (if and only if) the locks are not used in interrupts.
>
>So, (copied from the text itself):
>
>---
>The reasons you mustn't use these versions if you have interrupts that
>play with the spinlock is that you can get deadlocks:
>
>        spin_lock(&lock);
>        ...
>                <- interrupt comes in:
>                        spin_lock(&lock);
>---
>
>If you hold a spin lock without interrupts disabled, and an interrupt
>happens on the same CPU that holds the lock, and that interrupt handler
>tries to grab the lock it will just spin until that lock is released,
>which will _never_ happen, since the lock is held by the process that
>was interrupted, and will not run until the interrupt (that's spinning)
>is done. So you have a deadlock.
>
>Clear?
>
>-- Steve
>
>
>
>
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>  
>
Hi, Steven, Nice to meet you too ;) (These are the words that I first 
studied in English class.)

Yes, I am clear, I think I understand it in wrong direction before.

Good Luck.

-liyu