Re: BUG_ON(!cpus_equal(cpumask, tmp));

Hariprasad Nellitheertha
2004-03-31T04:43:40Z

There are actually two different problems here, and the BUG_ON is
hit in both cases.

1) In INIT_MM(), we now do this

.cpu_vm_mask = CPU_MASK_ALL,

With this, if we enter flush_tlb_others with init_mm, all bits except the one
corresponding to the current cpu are set. For example, on my UNI machine
running an SMP kernel with NR_CPUS=8, cpumask is 0xfe. The BUG_ON is hit
even if we are doing nothing related to shutdown or cpu-offlining (like
when we are just loading a new kernel into memory using kexec).

2) The issue of a race between taking CPUs offline and the tlbflush code. The
discussions have been centered around this issue.

The patch I sent across, though, is completely targetted towards issue 1.

Regards, Hari

On Tue, Mar 30, 2004 at 05:51:13PM -0800, Martin J. Bligh wrote:
> >> I think we're assuming that we don't have to because the problem is fixed
> >> by the "cpus_and(tmp, cpumask, cpu_online_map)" in flush_tlb_others so we
> >> don't have to. Except it's racy, and doesn't work.
> >
> > And it's a kludge, to work around dangling references to a CPU which has
> > gone away.
>
> Yes ;-)
>
> >> It would seem to me that your suggestion would fix it. But isn't locking
> >> cpu_online_map both simpler and (most importantly) more generic? I can't
> >> imagine that we don't use this elsewhere ... suppose for instance we took
> >> a timer interrupt, causing a scheduler rebalance, and moved a process to
> >> an offline CPU at that point? Isn't any user of smp_call_function also racy?
> >
> > If we have to add any fastpath locking to cope with CPU removal or reboot
> > then it's time to make CONFIG_HOTPLUG_CPU dependent upon CONFIG_BROKEN.
>
> Yeah, but as we've proved, it's not just hotplug, it's shutdown. And I don't
> think we can make that depend on CONFIG_BROKEN ;-) I don't see a *read*
> side RCU lock as an impostion on the fastpath (for reading cpu_online_map),
> and I don't care if writing to cpu_online_map is slower. A spinlock would
> be crappy, yes ...
>
> > yes, cpu_online_map should be viewed as a reference to the going-away CPU
> > for smp_call_function purposes. However the CPU takedown code appears to
> > do the right thing: it removes the cpu from cpu_online_map first, then does
> > the stop_machine() thing which should ensure that all other CPUs have
> > completed any cross-CPU call which they were doing, yes?
>
> Andy almost managed to convince me that the smp_call_function stuff is safe,
> based on call_lock exclusion. Except that we count that cpu stuff outside
> it ... but that's trivial to fix, we just move it inside the lock (patch
> below - untested, but trivial).
>
> He also pointed out that we could fairly easily fix the tlb stuff by
> taking the tlb lock before taking a cpu offline. Which still doesn't
> make me desperately comfortable ... but then he's smarter than me ;-)
> To me it comes down to ... do we want to lock the damned thing, or fix
> all the callers to be really, really careful?
>
> diff -purN -X /home/mbligh/.diff.exclude virgin/arch/i386/kernel/smp.c smp_call_function/arch/i386/kernel/smp.c
> --- virgin/arch/i386/kernel/smp.c 2004-03-11 14:33:36.000000000 -0800
> +++ smp_call_function/arch/i386/kernel/smp.c 2004-03-30 17:43:34.000000000 -0800
> @@ -514,10 +514,7 @@ int smp_call_function (void (*func) (voi
> */
> {
> struct call_data_struct data;
> - int cpus = num_online_cpus()-1;
> -
> - if (!cpus)
> - return 0;
> + int cpus;
>
> data.func = func;
> data.info = info;
> @@ -527,6 +524,10 @@ int smp_call_function (void (*func) (voi
> atomic_set(&data.finished, 0);
>
> spin_lock(&call_lock);
> + cpus = num_online_cpus()-1;
> + if (!cpus)
> + return 0;
> +
> call_data = &data;
> mb();
>
>

--
Hariprasad Nellitheertha
Linux Technology Center
India Software Labs
IBM India, Bangalore