On Wed, Jun 24, 2015 at 03:50:49PM +0200, Oleg Nesterov wrote: > On 06/23, Peter Zijlstra wrote: > > 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.
I'm confused.. why isn't the read-in-read recursion good enough? -- 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/
