Re: Deadlock between cpu_hotplug_begin and cpu_add_remove_lock

From: Srivatsa S. Bhat
Date: Thu Jan 23 2014 - 00:41:47 EST

On 01/23/2014 07:59 AM, Rusty Russell wrote:
> "Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> writes:
>> On 01/22/2014 02:00 PM, Srivatsa S. Bhat wrote:
>>> Hi Paul,
> I find an old patch for register_allcpu_notifier(), but the "bool
> replay_history" should be eliminated (always true): it's too weird.

Sorry, I didn't get this part. Why do you say that replay_history
will always be true?

replay_history will be set to true whenever the caller wants to
get notified of CPU_UP_PREPARE and CPU_ONLINE notifications for the
already online CPUs, or wants to run a custom setup-routine of its
own. And it will be false whenever the caller simply wants to just
register the callback.

Note that passing NULL for the setup-routine, by itself isn't enough
to make a decision. NULL + replay_history == True will invoke the normal
CPU_UP_PREPARE/CPU_ONLINE notifiers for the already online CPUs before
registering the callback. NULL + replay_history == False will just
register the callback and do nothing else.

> Then we should get rid of register_cpu_notifier, or at least hide it.

Why? Isn't it easier to use (since you don't have to pass 2 additional
parameters)? I see register_allcpu_notifier (or whatever better name
we can give it), as an API for special cases where there is something
more to be done than just registering the callback. And register_cpu_notifier
will continue to be the API for the regular case when the caller wants
to just register the callback. This latter case is the majority in the
kernel. So I don't think eliminating the regular API would be a good idea.

By the way, I'm still tempted to try out the simpler-looking alternative
idea of exporting cpu_maps_update_begin() and cpu_maps_update_done()
and then mandating that the callers do:

for_each_online_cpu(cpu) {

__register_cpu_notifier(); // this doesn't take the add_remove_lock

I'm working on a patchset that does this and performs a tree-wide
conversion. Please let me know if you have any objections to exporting
cpu_maps_update_begin/done() in this manner.

I thought I'd give this solution a try first, before going to the much
fancier register_allcpu_notifier() method.

Srivatsa S. Bhat

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