On Monday 10 June 2013 10:03:00 Alan Stern wrote:

[Thomas and Steve, please see the question below.]

On Mon, 10 Jun 2013, Ming Lei wrote:

That isn't clear.  It is documented that USB completion handlers are 
called with local interrupts disabled.  An IRQ arriving before the 
tasklet starts up might therefore be serviced during the short interval 
before the tasklet disables local interrupts, but if that happens it 
would mean that the completion handler would be delayed.

That is what tasklets do by definition, isn't it?
 

In effect, you are prioritizing other IRQs higher than USB completion
handlers.  Nevertheless, the total time spent with interrupts disabled
will remain the same.

It pobably slightly increases. You have colder caches twice.
And potentially you swich CPUs.
 

(There's one other side effect that perhaps you haven't considered.  
When multiple URBs are addressed to the same endpoint, their completion 
handlers are called in order of URB completion, which is the same as 
the order of URB submission unless an URB is cancelled.  By delegating 
the completion handlers to tasklets, and particularly by using per-CPU 
tasklets, you may violate this behavior.)

This is quite serious. It mustn't happen.

	Regards
		Oliver