Re: [Xen-devel] interrupt moving and NAPI scheduling

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Thu, 2014-05-15 at 17:26 +0100, Zoltan Kiss wrote:
> Hi,
> 
> Let me bump this question: how does the kernel make sure that if an 
> interrupt is moved away from CPU X to Y (e.g. by irqbalance), the NAPI 
> instance already scheduled on CPU Y won't race with the interrupt? I 
> mean the following scenario:
> 
> 1. instance calls napi_complete on CPU X
> 2. that removes the instance from the list
> 3. interrupt happens on CPU Y
> 4. "!test_and_set_bit(NAPI_STATE_SCHED, &n->state)" fails, as the bit is 
> still set
> 5. instance not added to the list therefore
> 6. napi_complete on CPU X clear the bit
> 
> My assumption is that when the interrupt moved, the kernel makes sure 
> the NAPI instance won't get scheduled on the old CPU, but I'm not sure 
> about it

Interrupts are supposed to be disabled at the time we call
napi_complete().

Some drivers are buggy.



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