Re: [PATCH 8/8 v2] Change irq_lock from mutex to spinlock.

[Avi Kivity <avi@redhat.com>]

On 08/12/2009 12:11 PM, Gleb Natapov wrote:
> On Wed, Aug 12, 2009 at 11:29:00AM +0300, Avi Kivity wrote:
>    
>> On 08/11/2009 03:31 PM, Gleb Natapov wrote:
>>      
>>> Change irq_lock from mutex to spinlock. We do not sleep while holding
>>> it.
>>>
>>>        
>> But why change?
>>
>>      
> Isn't it more lightweight? For the remaining use of the lock it doesn't
> really matters, but if I see mutex used somewhere I assume there are
> users that sleeps.
>    

Before the recent change, a mutex was more expensive if there was 
contention (waiter would schedule out).  Recently the mutex code was 
changed to spin while the holder was running.

>> The only motivation I can see is to allow injection from irqfd and
>> interrupt contexts without requiring a tasklet/work.  But that needs
>> spin_lock_irqsave(), not spin_lock().
>>
>>      
> After this series the lock is used only to protect modification of irq
> table, add/delete of ack notifiers and irq source id allocator. None of
> this affects injection from irqfd.
>
>    

Then it can be definitely left as mutex.

-- 
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