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

[Peter Ujfalusi]

On 02/06/2015 04:13 PM, Peter Zijlstra wrote:
> 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);

Certainly looks much simpler, but it adds quite a bit of data to the
omap_hwmod struct, and we have a _lot_ of them for omap2plus configuration.

ls -al vmlinux

w/o any the lockdep warning fixes:
110109168

With my series applied:
110112031 (base + 2863)

With setting individual lockdep class:
110114275 (base + 5107)

I certainly like the lockdep_set_class() way since it is cleaner, but it adds
almost double amount of bytes to the kernel.
I'll test the patch on my board tomorrow as first thing.

-- 
Péter
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