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SubjectRe: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
On Thu, Nov 30, 2006 at 12:46:17PM +0100, Ingo Molnar wrote:
>
> struct task_struct {
> ...
> int hotplug_depth;
> struct mutex *hotplug_lock;
> }
> ...
>
> DEFINE_PER_CPU(struct mutex, hotplug_lock);
>
> void cpu_hotplug_lock(void)
> {
> int cpu = raw_smp_processor_id();
> /*
> * Interrupts/softirqs are hotplug-safe:
> */
> if (in_interrupt())
> return;
> if (current->hotplug_depth++)
> return;
> current->hotplug_lock = &per_cpu(hotplug_lock, cpu);
> mutex_lock(current->hotplug_lock);
> }
>
> void cpu_hotplug_unlock(void)
> {
> int cpu;
>
> if (in_interrupt())
> return;
> if (DEBUG_LOCKS_WARN_ON(!current->hotplug_depth))
> return;
> if (--current->hotplug_depth)
> return;
>
> mutex_unlock(current->hotplug_lock);
> current->hotplug_lock = NULL;
> }
>

In process context preemptible code,
Lets say we are currently running on processor i.

cpu_hotplug_lock() ; /* does mutex_lock(&percpu(hotplug_lock, i)) */

/* do some operation, which might sleep */
/* migrates to cpu j */

cpu_hotplug_unlock(); /* does mutex_unlock(&percpu(hotplug_lock, i)
while running on cpu j */

This would cause cacheline ping pong, no?

>
> Ingo

regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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