On Monday, 14 May 2007 09:26, Gautham R Shenoy wrote:
> On Mon, May 14, 2007 at 11:48:46AM +0530, Srivatsa Vaddagiri wrote:
> >
> > The other complication get/put_hotcpu() had was dealing with
> > write-followed-by-read lock attempt by the *same* thread (whilst doing
> > cpu_down/up). IIRC this was triggered by some callback processing in CPU_DEAD
> > or CPU_DOWN_PREPARE.
> >
> >
> > cpu_down()
> > |- take write lock
> > |- CPU_DOWN_PREPARE
> > | |- foo() wants a read_lock
> >
> > Stupid as it sounds, it was really found to be happening! Gautham, do you
> > recall who that foo() was? Somebody in cpufreq I guess ..
>
> IIRC, it was a problem with ondemand. while handling CPU_DEAD, ondemand code
> would call destroy_workqueue, which tried flushing the workqueue, which
> once upon a time did lock_cpu_hotplug, before Oleg and Andrew cleaned
> that up.
>
> Ofcourse, cpufreq works fine now after Venki's patches which
> just nullifies the reference to the policy structure of the cpu to be
> removed during the CPU_DOWN_PREPARE by calling __cpufreq_remove_dev
> instead of handling it in CPU_DEAD.
>
> However, as we have discovered, without freezing all the threads, it
> is inadvisable to call flush_workqueue from a cpu-hotplug callback
> path.
Please see my recent patch at http://lkml.org/lkml/2007/5/14/7 .
It's not exactly the same thing, but I think the trick in there might be
useful.
Greetings,
Rafael
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