Quoting Thomas Gleixner (2013-03-27 04:24:12)
On Wed, 27 Mar 2013, Mike Turquette wrote:
+/*** locking & reentrancy ***/
+static void clk_fwk_lock(void)
This function name sucks as much as the whole implementation does.
+{
/* hold the framework-wide lock, context == NULL */mutex_lock(&prepare_lock);/* set context for any reentrant calls */atomic_set(&prepare_context, (int) get_current());And what's the point of the atomic here? There is no need for an atomic if you hold the lock. Neither here nor on the reader side.
I had wondered about that. So the barriers in mutex_lock and spin_lock_irqsave are sufficient such that the (unprotected) read-side will always see the correct data? That makes sense to me since accesses to the clock tree are still serialized.
Aside of that, the cast to (int) and the one below to (void *) are blantantly wrong on 64 bit.
Since the atomic type is no longer required (based on the above assumption) then this problem goes away. Each context is just a global pointer.
+}
+static void clk_fwk_unlock(void) +{
/* clear the context */atomic_set(&prepare_context, 0);/* release the framework-wide lock, context == NULL */mutex_unlock(&prepare_lock);+}
+static bool clk_is_reentrant(void) +{
if (mutex_is_locked(&prepare_lock))if ((void *) atomic_read(&prepare_context) == get_current())Mooo.
Woof?
return true;return false;+}
Why the heck do you need this function?
Just to sprinkle all these ugly constructs into the code:
mutex_lock(&prepare_lock);
/* re-enter if call is from the same context */if (clk_is_reentrant()) {__clk_unprepare(clk);return;}Sigh. Why not doing the obvious?
Step 1/2: Wrap locking in helper functions
+static void clk_prepare_lock(void) +{
mutex_lock(&prepare_lock);+}
+static void clk_prepare_unlock(void) +{
mutex_unlock(&prepare_lock);+}
That way the whole change in the existing code boils down to:
mutex_lock(&prepare_lock);
clk_prepare_lock();...
mutex_unlock(&prepare_lock);
clk_prepare_unlock();Ditto for the spinlock.
And there is no pointless reshuffling of functions required.
Step 2/2: Implement reentrancy
+static struct task_struct *prepare_owner; +static int prepare_refcnt;
static void clk_prepare_lock() {
mutex_lock(&prepare_lock);
if (!mutex_trylock(&prepare_lock)) {if (prepare_owner == current) {prepare_refcnt++;return;}mutex_lock(&prepare_lock);}WARN_ON_ONCE(prepare_owner != NULL);WARN_ON_ONCE(prepare_refcnt != 0);prepare_owner = current;prepare_refcnt = 1;}
static void clk_prepare_unlock(void) {
mutex_unlock(&prepare_lock);
WARN_ON_ONCE(prepare_owner != current);WARN_ON_ONCE(prepare_refcnt == 0);if (--prepare_refcnt)return;prepare_owner = NULL;mutex_unlock(&prepare_lock);}
Ditto for the spinlock.
That step requires ZERO change to the functions. They simply work and you don't need all this ugly reentrancy hackery.
Thanks for the review Thomas. I will steal your code and call it my own in the next version. In particular getting rid of the atomics makes things much nicer.
Regards, Mike
Thanks,
tglx