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
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