Messages in this thread | | | Subject | Re: [PATCH] generic-smp: remove kmalloc() | From | Peter Zijlstra <> | Date | Tue, 17 Feb 2009 18:40:20 +0100 |
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On Tue, 2009-02-17 at 18:21 +0100, Oleg Nesterov wrote: > On 02/17, Peter Zijlstra wrote: > > > > Ok, so this is on top of Nick's cleanup from earlier today, and folds > > everything. > > > > No more RCU games as the storage for per-cpu entries is permanent - cpu > > hotplug should be good because it does a synchronize_sched(). > > > > What we do play games with is the global list, we can extract entries > > and place them to the front while its being observed. This means that > > the list iteration can see some entries twice (not a problem since we > > remove ourselves from the cpumask), but cannot miss entries. > > I think this all is correct.
*phew* :-)
> But I am wondering, don't we have another problem. Before this patch, > smp_call_function_many(wait => 0) always succeeds, no matter which > locks the caller holds. > > After this patch we can deadlock, csd_lock() can spin forever if the > caller shares the lock with another func in flight. > > IOW, > void func(void *arg) > { > lock(LOCK); > unlock(LOCK); > } > > CPU 0 does: > > smp_call_function(func, NULL, 0); > lock(LOCK); > smp_call_function(another_func, NULL, 0); > unlock(LOCK); > > If CPU 0 takes LOCK before CPU 1 calls func, the 2nd smp_call_function() > hangs in csd_lock(). > > I am not sure this is the real problem (even if I am right), perhaps > the answer is "don't do that". > > But, otoh, afaics we can tweak generic_smp_call_function_interrupt() > a bit to avoid this problem. Something like > > list_for_each_entry_rcu(data, &call_function.queue, csd.list) { > void (*func)(void *); > void *info; > int refs; > > spin_lock(&data->lock); > if (!cpumask_test_cpu(cpu, data->cpumask)) { > spin_unlock(&data->lock); > continue; > } > cpumask_clear_cpu(cpu, data->cpumask); > WARN_ON(data->refs == 0); > refs = --data->refs; > func = data->csd.func; > info = data->csd.info; > wait = (data->flags & CSD_FLAG_WAIT); > spin_unlock(&data->lock); > > if (!refs) { > spin_lock(&call_function.lock); > list_del_rcu(&data->csd.list); > spin_unlock(&call_function.lock); > csd_unlock(&data->csd); > } > > func(info); > if (!refs && wait) > csd_complete(&data->csd); > } > > I am afraid I missed something, and the code above looks wrong > because it does csd_unlock() first, then csd_complete().
That does look a bit weird, but
> But if wait == T, then nobody can reuse this per-cpu entry, the > caller of smp_call_function_many() must spin in csd_wait() on > the same CPU.
is indeed correct.
> What do you think?
While I would say, don't do that to your deadlock scenario, I do like the extra freedom this provides, so I'm inclined to go with this. Let me spin a new patch and build a kernel with it ;-)
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