Re: [PATCH 0/2] ARM: omap2+: omap_hwmod: Fix false lockdep warning

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 06, 2015 at 02:48:34PM +0200, Peter Ujfalusi wrote:
> Hi,
> 
> In case when hwmods are used in nested way the lockdep validator will print out
> a warning message about possible deadlock situation:
> 
> [    4.514882] =============================================
> [    4.520530] [ INFO: possible recursive locking detected ]
> [    4.526176] 3.14.30-00289-ge44872fdca8f-dirty #198 Not tainted
> [    4.532285] ---------------------------------------------
> [    4.537936] kworker/u4:1/18 is trying to acquire lock:
> [    4.543317]  (&(&oh->_lock)->rlock){......}, at: [<c002d2dc>] omap_hwmod_enable+0x2c/0x58
> [    4.552109] 
> [    4.552109] but task is already holding lock:
> [    4.558216]  (&(&oh->_lock)->rlock){......}, at: [<c002d2dc>] omap_hwmod_enable+0x2c/0x58
> [    4.566999] 
> [    4.566999] other info that might help us debug this:
> [    4.573831]  Possible unsafe locking scenario:
> [    4.573831] 
> [    4.580025]        CPU0
> [    4.582584]        ----
> [    4.585142]   lock(&(&oh->_lock)->rlock);
> [    4.589544]   lock(&(&oh->_lock)->rlock);
> [    4.593945] 
> [    4.593945]  *** DEADLOCK ***
> [    4.593945] 
> [    4.600146]  May be due to missing lock nesting notation
> 
> What lockdep did not realizes is that the two oh is not the same hwmod object
> and we have taken different locks.
> 
> One example of nested hwmod usage is on DRA7xx platforms, where McASP can be
> configured to use ATL clock as it's functional clock. In this case the
> pm_runtime_get/put_sync call will enable the mcasp hwmod and as part of this
> operation it will enable the needed clocks. Since ATL clock is needed, we will
> have another pm_runtime operation from the ATL clock enable callback which
> will take the atl hwmod's lock. This will be seen by lockdep as possible
> deadlock situation.
> 
> In order to notify lockdep about this, we need to switch using _nested
> version of locking function and assign different subclass to those hwmods which
> could be used in nested way.

IFF struct omap_hwmod is always statically allocated; as I think the
__init on _register() mandates, you can use the below annotation.

---
 arch/arm/mach-omap2/omap_hwmod.c | 1 +
 arch/arm/mach-omap2/omap_hwmod.h | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/arm/mach-omap2/omap_hwmod.c b/arch/arm/mach-omap2/omap_hwmod.c
index 9025ffffd2dc..222d654ad6fd 100644
--- a/arch/arm/mach-omap2/omap_hwmod.c
+++ b/arch/arm/mach-omap2/omap_hwmod.c
@@ -2698,6 +2698,7 @@ static int __init _register(struct omap_hwmod *oh)
 	INIT_LIST_HEAD(&oh->master_ports);
 	INIT_LIST_HEAD(&oh->slave_ports);
 	spin_lock_init(&oh->_lock);
+	lockdep_set_class(&oh->_lock, &oh->hwmod_key);
 
 	oh->_state = _HWMOD_STATE_REGISTERED;
 
diff --git a/arch/arm/mach-omap2/omap_hwmod.h b/arch/arm/mach-omap2/omap_hwmod.h
index 5b42fafcaf55..754bdfb3f811 100644
--- a/arch/arm/mach-omap2/omap_hwmod.h
+++ b/arch/arm/mach-omap2/omap_hwmod.h
@@ -689,6 +689,8 @@ struct omap_hwmod {
 	u8				_state;
 	u8				_postsetup_state;
 	struct omap_hwmod		*parent_hwmod;
+
+	struct lock_class_key		hwmod_key;
 };
 
 struct omap_hwmod *omap_hwmod_lookup(const char *name);