On Mon, 20 May 2002, David S. Miller wrote:
> Such rule does not even make this piece of code legal. Consider:
>
> task1:cpu0: x = counters[smp_processor_id()];
> cpu0: PREEMPT
> task2:cpu0: x = counters[smp_processor_id()];
> task2:cpu0: counters[smp_processor_id()] = x + 1;
> cpu0: PREEMPT
> task1:cpu0: counters[smp_processor_id()] = x + 1;
> full garbage
>
> But it does bring up important point, preemption people need to
> fully audit entire networking.
>
> It is totally broken by preemption the more I think about it.
>
> At the very beginning, all the SNMP counter bumping tricks will
> totally fail with preemption enabled.
>
A lot of the synchronization between process context and interrupt
context is based on per-cpu data structures or simple locks
(without disabling irq's globally) eg:
softnet_data queue (we only disable local interrupts), and
synchronization between tcp_readmsg() and tcp_rcv() over
the receive queue would get confused (lock.users flag would
be different on another CPU)..
Wonder how any of it could possibly work..
thanks,
Nivedita
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