Re: softirq buggy [Re: Serial port latency]

Manfred Spraul
2001-04-04T21:19:08Z

From: "Pavel Machek" <pavel@ucw.cz>
>
> > Ok, there are 2 bugs that are (afaics) impossible to fix without
> > checking for pending softirq's in cpu_idle():
> >
> > a)
> > queue_task(my_task1, tq_immediate);
> > mark_bh();
> > schedule();
> > ;within schedule: do_softirq()
> > ;within my_task1:
> > mark_bh();
> > ; bh returns, but do_softirq won't loop
> > ; do_softirq returns.
> > ; schedule() clears current->need_resched
> > ; idle thread scheduled.
> > --> idle can run although softirq's are pending
>
> Or anything else can run altrough softirqs are pending. If it is
> computation job, softinterrupts are delayed quiet a bit, right?
>
> So right fix seems to be "loop in do_softirq".
>
No, it's the wrong fix.
A network server under high load would loop forever within the softirq,
never returning to process level.

do_softirq cannot loop, the right fix is "check often for pending
softirq's".
It's checked before a process returns to user space, it's checked when a
process schedules. What's missing is that the idle functions must check
for pending softirqs, too.

--
Manfred