Messages in this thread | | | Subject | Re: [PATCH 5/8] cpu: cpu-hotplug deadlock | From | Peter Zijlstra <> | Date | Tue, 29 Apr 2008 19:31:56 +0200 |
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On Tue, 2008-04-29 at 20:45 +0400, Oleg Nesterov wrote: > 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" ?
Darn, now you make me think ;-)
Ok, I think I have it; the crux of the matter is that we want reader-in-writer recursion for the cpu hotplug lock.
So we want:
cpu_hotplug.write_lock() A.lock() cpu_hotplug.read_lock()
When - as it was - the write lock is implemented as keeping the lock internal lock (the lock guarding the lock state) locked over the entire write section, and the read lock side is, LOCK; change state; UNLOCK, the above will result in a deadlock like:
C.lock A.lock C.lock
By making both the read and write side work like:
LOCK change state UNLOCK
the internal lock will not deadlock.
So what I did was promote cpu_hotplug to a full lock that handled read-in-read and read-in-write recursion and made cpu_hotplug.lock the lock internal lock.
> > > (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.
I don't think we do. It just didn't occur to me to pile read-waiters and write-waiters on the same waitqueue.
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