Re: [PATCH 5/8] cpu: cpu-hotplug deadlock

[Oleg Nesterov <oleg@tv-sign.ru>]

On 04/29, Peter Zijlstra wrote:
>
> The only thing that changed is that the mutex is not held; so what we
> change is:
> 
>  LOCK
> 
>  ... do the full hotplug thing ...
> 
>  UNLOCK
> 
> into
> 
>  LOCK
>   set state
>  UNLOCK
> 
>  ... do the full hotplug thing ...
> 
>  LOCK
>   unset state
>  UNLOCK
> 
> So that the lock isn't held over the hotplug operation.

Well, yes I see, but... Ugh, I have a a blind spot here ;)

why this makes any difference from the semantics POV ? why it is bad
to hold the mutex throughout the "full hotplug thing" ?

> > (actually, since write-locks should be very rare, perhaps we don't need
> >  2 wait_queues ?)
> 
> And just let them race the wakeup race, sure that might work. Gautham
> even pointed out that it never happens because there is another
> exclusive lock on the write path.
> 
> But you say you like that it doesn't depend on that anymore - me too ;-)

Yes. but let's suppose we have the single wait_queue, this doesn't make
any difference from the correctness POV, no?

To clarify: I am not arguing! this makes sense, but I'm asking to be sure
I didn't miss a subtle reason why do we "really" need 2 wait_queues.

Also. Let's suppose we have both read- and write- waiters, and cpu_hotplug_done()
does wake_up(writer_queue). It is possible that another reader comes and does
get_online_cpus() and increments .refcount first. After that, cpu_hotplug
is "opened" for the read-lock, but other read-waiters continue to sleep, and
the final put_online_cpus() wakes up write-waiters only. Yes, this all is
correct, but not "symmetrical", and leads to the question "do we really need
2 wait_queues" again.

Oleg.