[Xen-devel] question on the use of fs in evtchn.c

[ron minnich <rminnich@xxxxxxxx>]

       while ( (l1i = ffs(l1)) != 0 )
                               ^^
        {
            l1i--;
            l1 &= ~(1 << l1i);

            l2 = s->evtchn_pending[l1i] & ~s->evtchn_mask[l1i];
            while ( (l2i = ffs(l2)) != 0 )
                                    ^^
            {
                l2i--;
                l2 &= ~(1 << l2i);

                port = (l1i << 5) + l2i;

this is in:
"../linux-2.4.26-xenU/arch/xen/kernel/evtchn.c" 

The problem is that ffs will return -1 if l1 is 0. Yeah, I know, unlikely,
but if it ever happens that l1 or l2 are 0, well, you're in for a fun
time. In any event, the loop condition holds for both a valid result and 
an error, which may not be what you really want to do. 

In the plan 9 port this is:

       while ( (l1i = ffs(l1)) > 0 )
                               ^
        {
            l1i--;
            l1 &= ~(1 << l1i);

            l2 = s->evtchn_pending[l1i] & ~s->evtchn_mask[l1i];
            while ( (l2i = ffs(l2)) > 0 )
                                    ^
            {
                l2i--;
                l2 &= ~(1 << l2i);

                port = (l1i << 5) + l2i;

A little paranoid, I realize, but mistakes happen. I can't think of a case 
where this would happen, but what the heck. Maybe if l1 is 0 it should 
complain ...

BTW my little patch for level (not edge) VIRQ_TIMER interrupts is giving
me something more like what I expect from timer interrupts. I hope you'll
at least consider the idea.

ron


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