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

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Date:	Sat, 29 Jul 2006 15:45:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ashok Raj <ashok.raj@...el.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Dave Jones <davej@...hat.com>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [patch] Reorganize the cpufreq cpu hotplug locking to not be totally bizare


* Ashok Raj <ashok.raj@...el.com> wrote:

> On Thu, Jul 27, 2006 at 03:40:49AM +0200, Ingo Molnar wrote:
> > 
> > * Linus Torvalds <torvalds@...l.org> 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.

	Ingo
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