Re: [Xen-devel] fooey. no interrupts.

[ron minnich <rminnich@xxxxxxxx>]

I've got a question about this code. 


static inline void evtchn_set_pending(struct domain *d, int port)
{
    shared_info_t *s = d->shared_info;
    if ( !test_and_set_bit(port,    &s->evtchn_pending[0]) &&
         !test_bit        (port,    &s->evtchn_mask[0])    &&
         !test_and_set_bit(port>>5, &s->evtchn_pending_sel) )
    {
        /* The VCPU pending flag must be set /after/ update to 
                        evtchn-pend. */
        s->vcpu_data[0].evtchn_upcall_pending = 1;
        guest_async_callback(d);
    }
}

So you'll get an upcall IFF: the bit 'port' in evtchn_pending WAS 0, the 
bit 'port' in the mask IS 0, and the bit 'port >> 5' in the 
evtchn_pending_sel WAS 0. 

OK, here's my question: suppose the first test_and_set_bit fails because
the bit in evtchn_pending[0] was already set? You'll never get called,
that's what, as far as I can tell. And this is exactly what I'm seeing.  
I've got bits 0,1,2 set in evtchn_pending, but the guest_async_callback is
never happening, since the test_and_set_bit returns 1.  I'm missing an
interrupt, due to a plethora of debug prints in my kernel, and I'm not
seeing another one.

To me, it looks like I'm exercising a race condition in this function 
shown above. 

Here is my question: why isn't this code something like:

static inline void evtchn_set_pending(struct domain *d, int port)
{
    shared_info_t *s = d->shared_info;
    set_bit(port, &s->evtchn_pending[0]);
    if ( !test_bit        (port,    &s->evtchn_mask[0])    &&
         !test_and_set_bit(port>>5, &s->evtchn_pending_sel) )
    {
        /* The VCPU pending flag must be set /after/ update to 
                evtchn-pend. */
        s->vcpu_data[0].evtchn_upcall_pending = 1;
        guest_async_callback(d);
    }
}

In other words, I don't see the reason for the first test_and_set_bit, 
given that the bit may have been set by an earlier call to 
evtchn_set_pending, masked by the mask, and then the next time you call 
the first test_and_set_bit will fail. 

So, what's the reason for that first TAS? 

thanks

ron

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