Re: [RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 16 Oct 2007, Gautham R Shenoy wrote:
> 
> Patch 1/4: Implements the core refcount + waitqueue model.
> Patch 2/4: Replaces all the lock_cpu_hotplug/unlock_cpu_hotplug instances
> 	   with get_online_cpus()/put_online_cpus()
> Patch 3/4: Replaces the subsystem mutexes (we do have three of them now, 
>            in sched.c, slab.c and workqueue.c) with get_online_cpus,
> 	   put_online_cpus.
> Patch 4/4: Eliminates the CPU_DEAD and CPU_UP_CANCELLED event handling
>  	   from workqueue.c
> 
> The patch series has survived an overnight test with kernbench on i386.
> and has been tested with Paul Mckenney's latest preemptible rcu code.
> 
> Awaiting thy feedback!

Well, afaik, the patch series is fairly clean, and I'm obviously perfectly 
happy with the approach, so I have no objections. 

But it looks buggy. This:

	+static void cpu_hotplug_begin(void)
	+{
	+       mutex_lock(&cpu_hotplug.lock);
	+       cpu_hotplug.active_writer = current;
	+       while (cpu_hotplug.refcount) {
	+               mutex_unlock(&cpu_hotplug.lock);
	+               wait_for_completion(&cpu_hotplug.readers_done);
	+               mutex_lock(&cpu_hotplug.lock);
	+       }
	+
	+}

drops the cpu_hotplug.lock, which - as far as I can see - means that 
another process can come in and do the same, and mess up the 
"active_writer" thing. The oerson that actually *gets* the lock may not be 
the same one that has "active_writer" set to itself. No? Am I missing 
something.

So I think this needs (a) more people looking at it (I think I found a 
bug, who knows if there are more subtle ones lurking) and (b) lots of 
testing. 

			Linus