lkml.org 
[lkml]   [2007]   [Oct]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.


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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-10-16 19:25    [W:0.159 / U:0.332 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site