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SubjectRe: Semphore -> mutex in the device tree
From
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On Thu, 2008-04-17 at 11:22 -0400, Alan Stern wrote:
> Peter:
>
> The obstacle to converting the semaphore in struct device to a mutex
> has been that its tree-oriented usage pattern isn't compatible with
> lockdep.
>
> In order to get around this and at least begin the conversion process,
> how about adding a provision for making some classes of mutex invisible
> to lockdep? I know it doesn't solve the fundamental problem, but maybe
> it's a step in the right direction.

the device lock has two problems with lockdep:

1) on suspend it takes more than MAX_LOCK_DEPTH (48) locks

2) tree nesting


Lets start with the easy one first; would a similar solution to the
radix tree locking as found in -rt work?

http://programming.kicks-ass.net/kernel-patches/concurrent-pagecache/23-rc1-rt/radix-concurrent-lockdep.patch

That does mean you have to set an effective max depth to the tree, is
that a practical issue?

The harder part is 1), holding _that_ many locks. Would something
obscene like this work for you:


struct device_suspend {
wait_queue_head_t wait_queue;
struct srcu_struct srcu;
int suspend;
} dev_suspend_state;

void device_lock(struct device *dev)
{
again:
srcu_read_lock(&dev_suspend_state.srcu);
if (unlikely(rcu_dereference(dev_suspend_state.suspend))) {
srcu_read_unlock(&dev_suspend_state.srcu);
wait_event(&dev_suspend_state.wait_queue,
!dev_suspend_state.suspend);
goto again;
}
mutex_lock(&dev->mutex);
}

void device_unlock(struct device *dev)
{
mutex_unlock(&dev->mutex);
srcu_read_unlock(&dev_suspend_state.srcu);
}


void device_suspend(void)
{
rcu_assign_pointer(dev_suspend_state.suspend, 1);
synchronize_srcu(&dev_suspend_state.srcu);
}

void device_resume(void)
{
rcu_assign_pointer(dev_suspend_state.suspend, 0);
wake_up_all(&dev_suspend_state.wait_queue);
}






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