Re: [patch] Reorganize the cpufreq cpu hotplug locking to not be totally bizare

From: Ingo Molnar
Date: Sat Jul 29 2006 - 09:50:58 EST

* Ashok Raj <ashok.raj@xxxxxxxxx> wrote:

> On Thu, Jul 27, 2006 at 03:40:49AM +0200, Ingo Molnar wrote:
> >
> > * Linus Torvalds <torvalds@xxxxxxxx> wrote:
> >
> > > But I agree with Arjan - I think the fundamental problem is that cpu
> > > hotplug locking is just is fundamentally badly designed as-is. There's
> > > really very little point to making it a _lock_ per se, since most
> > > people really want more of a "I'm using this CPU, don't try to remove
> > > it right now" thing which is more of a ref-counting-like issue.
> >
> > we'd also need a facility to wait on that refcount - i.e. a waitqueue.
> > Which means we'd have a "refcount + waitqueue", which is equivalent to a
> > "recursive, sleeping read-lock", where the write-side could be used as a
> > simple facility to "wait for all readers to go away and block new
> > readers from entering the critical sections". [which type of lock Linux
> > does not have right now. rwsems come the closest but they dont recurse.]
> sounds like some varient of conditional variables, caveat might be
> that new readers permitted if in the same call thread/cpu?

well, i'd just call it a recursive rwsem. (sure, you can express it via
condition variables, but just about any locking method can be expressed
via them.)

> > Also, the hotplug lock is global right now which is pretty unscalable,
> > so the rw-mutex should also be per-CPU, and the hotplug locking API
> > should be changed to something like:
> >
> > cpu = cpu_hotplug_lock();
> so this is sort of like the get_cpu()/put_cpu() interface that does
> preempt_disable() + get current cpu.

the API is similar - behavior is different in that the 'per-cpu lock'
i'm talking about _does_ allow preemption and migration to another CPU.

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