On 06/23, Peter Zijlstra wrote: > > On Tue, Jun 23, 2015 at 07:01:22PM +0200, Oleg Nesterov wrote: > > On 06/23, Peter Zijlstra wrote: > > > > > > On Tue, Jun 23, 2015 at 12:57:39AM +0200, Oleg Nesterov wrote: > > > > > + > > > > > + lock_map_acquire_read(&cpu_hotplug.rwsem.rw_sem.dep_map); > > > > > + _percpu_down_read(&cpu_hotplug.rwsem); > > > > > } > > > > > > > > Confused... Why do we need _percpu_down_read()? Can't get_online_cpus() > > > > just use percpu_down_read() ? > > > > > > > > Yes, percpu_down_read() is not recursive, like the normal down_read(). > > > > But this does not matter because we rely on ->cpuhp_ref anyway? > > > > > > While we will not call the actual lock, lockdep will still get confused > > > by the inconsistent locking order observed. > > > > > > Change it and boot, you'll find lockdep output pretty quickly. > > > > Hmm. and I simply can't understand why... > > If in one callchain we do: > > get_online_cpus(); > lock(A); > > in another we do: > > lock(A); > get_online_cpus(); > > lockdep will complain about the inverted lock order, however this is not > a problem at all for recursive locks.
Ah, but in this case lockdep is right. This is deadlockable because with the new implementation percpu_down_write() blocks the new readers. So this change just hides the valid warning. Just suppose that the 3rd CPU does percpu_down_write()->down_write() right after the 2nd CPU (above) takes lock(A). I have to admit that I didn't realize that the code above is currently correct... but it is. So we need percpu_down_write_dont_block_readers(). I already thought about this before, I'll try to make the patch tomorrow on top of your changes. This means that we do not need task_struct->cpuhp_ref, but we can't avoid livelock we currently have: cpu_hotplug_begin() can never succeed if the new readers come fast enough. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
