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Message-ID: <alpine.LFD.0.999.0710161015510.6887@woody.linux-foundation.org>
Date: Tue, 16 Oct 2007 10:20:37 -0700 (PDT)
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Gautham R Shenoy <ego@...ibm.com>
cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org,
Srivatsa Vaddagiri <vatsa@...ibm.com>,
Rusty Russel <rusty@...tcorp.com.au>,
Dipankar Sarma <dipankar@...ibm.com>,
Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...e.hu>,
Paul E McKenney <paulmck@...ibm.com>
Subject: Re: [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
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