Messages in this thread | | | Subject | Re: Semphore -> mutex in the device tree | From | Peter Zijlstra <> | Date | Thu, 17 Apr 2008 17:45:53 +0200 |
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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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