Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master

From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
To: Daniel Vetter <daniel@ffwll.ch>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-graphics-maintainer@vmware.com, zackr@vmware.com,
	airlied@linux.ie, maarten.lankhorst@linux.intel.com,
	mripard@kernel.org, tzimmermann@suse.de
Cc: dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
	skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
	linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master
Date: Thu, 29 Jul 2021 22:32:13 +0800	[thread overview]
Message-ID: <fbf2ec46-5ef5-7108-450a-13a7c48c30ce@gmail.com> (raw)
In-Reply-To: <YQJSE3TMRydDNhqT@phenom.ffwll.local>

On 29/7/21 3:00 pm, Daniel Vetter wrote:
> On Tue, Jul 27, 2021 at 04:37:22PM +0200, Peter Zijlstra wrote:
>> On Thu, Jul 22, 2021 at 12:38:10PM +0200, Daniel Vetter wrote:
>>> On Thu, Jul 22, 2021 at 05:29:27PM +0800, Desmond Cheong Zhi Xi wrote:
>>>> Inside drm_is_current_master, using the outer drm_device.master_mutex
>>>> to protect reads of drm_file.master makes the function prone to creating
>>>> lock hierarchy inversions. Instead, we can use the
>>>> drm_file.master_lookup_lock that sits at the bottom of the lock
>>>> hierarchy.
>>>>
>>>> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>>>> ---
>>>>   drivers/gpu/drm/drm_auth.c | 9 +++++----
>>>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>>>> index f00354bec3fb..9c24b8cc8e36 100644
>>>> --- a/drivers/gpu/drm/drm_auth.c
>>>> +++ b/drivers/gpu/drm/drm_auth.c
>>>> @@ -63,8 +63,9 @@
>>>>   
>>>>   static bool drm_is_current_master_locked(struct drm_file *fpriv)
>>>>   {
>>>> -	lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
>>>> -
>>>> +	/* Either drm_device.master_mutex or drm_file.master_lookup_lock
>>>> +	 * should be held here.
>>>> +	 */
>>>
>>> Disappointing that lockdep can't check or conditions for us, a
>>> lockdep_assert_held_either would be really neat in some cases.
>>>
>>> Adding lockdep folks, maybe they have ideas.
>>
>> #ifdef CONFIG_LOCKDEP
>> 	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&drm_device.master_mutex) ||
>> 				      lockdep_is_held(&drm_file.master_lookup_lock)));
>> #endif
>>
>> doesn't exactly roll off the tongue, but should do as you want I
>> suppose.
>>
>> Would something like:
>>
>> #define lockdep_assert(cond)	WARN_ON_ONCE(debug_locks && !(cond))
>>
>> Such that we can write:
>>
>> 	lockdep_assert(lockdep_is_held(&drm_device.master_mutex) ||
>> 		       lockdep_is_held(&drm_file.master_lookup_lock));
>>
>> make it better ?
> 
> Yeah I think that's pretty tidy and flexible.
> 
> Desmond, can you pls give this a shot with Peter's patch below?
> -Daniel

Sounds good, will do. Thanks for the patch, Peter.

Just going to make a small edit:
s/LOCK_STAT_NOT_HELD/LOCK_STATE_NOT_HELD/

Best wishes,
Desmond

>>
>> ---
>> Subject: locking/lockdep: Provide lockdep_assert{,_once}() helpers
>>
>> Extract lockdep_assert{,_once}() helpers to more easily write composite
>> assertions like, for example:
>>
>> 	lockdep_assert(lockdep_is_held(&drm_device.master_mutex) ||
>> 		       lockdep_is_held(&drm_file.master_lookup_lock));
>>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> ---
>> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
>> index 5cf387813754..0da67341c1fb 100644
>> --- a/include/linux/lockdep.h
>> +++ b/include/linux/lockdep.h
>> @@ -306,31 +306,29 @@ extern void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie);
>>   
>>   #define lockdep_depth(tsk)	(debug_locks ? (tsk)->lockdep_depth : 0)
>>   
>> -#define lockdep_assert_held(l)	do {					\
>> -		WARN_ON(debug_locks &&					\
>> -			lockdep_is_held(l) == LOCK_STATE_NOT_HELD);	\
>> -	} while (0)
>> +#define lockdep_assert(cond)		\
>> +	do { WARN_ON(debug_locks && !(cond)); } while (0)
>>   
>> -#define lockdep_assert_not_held(l)	do {				\
>> -		WARN_ON(debug_locks &&					\
>> -			lockdep_is_held(l) == LOCK_STATE_HELD);		\
>> -	} while (0)
>> +#define lockdep_assert_once(cond)	\
>> +	do { WARN_ON_ONCE(debug_locks && !(cond)); } while (0)
>>   
>> -#define lockdep_assert_held_write(l)	do {			\
>> -		WARN_ON(debug_locks && !lockdep_is_held_type(l, 0));	\
>> -	} while (0)
>> +#define lockdep_assert_held(l)		\
>> +	lockdep_assert(lockdep_is_held(l) != LOCK_STAT_NOT_HELD)
>>   
>> -#define lockdep_assert_held_read(l)	do {				\
>> -		WARN_ON(debug_locks && !lockdep_is_held_type(l, 1));	\
>> -	} while (0)
>> +#define lockdep_assert_not_held(l)	\
>> +	lockdep_assert(lockdep_is_held(l) != LOCK_STATE_HELD)
>>   
>> -#define lockdep_assert_held_once(l)	do {				\
>> -		WARN_ON_ONCE(debug_locks && !lockdep_is_held(l));	\
>> -	} while (0)
>> +#define lockdep_assert_held_write(l)	\
>> +	lockdep_assert(lockdep_is_held_type(l, 0))
>>   
>> -#define lockdep_assert_none_held_once()	do {				\
>> -		WARN_ON_ONCE(debug_locks && current->lockdep_depth);	\
>> -	} while (0)
>> +#define lockdep_assert_held_read(l)	\
>> +	lockdep_assert(lockdep_is_held_type(l, 1))
>> +
>> +#define lockdep_assert_held_once(l)		\
>> +	lockdep_assert_once(lockdep_is_held(l) != LOCK_STAT_NOT_HELD)
>> +
>> +#define lockdep_assert_none_held_once()		\
>> +	lockdep_assert_once(!current->lockdep_depth)
>>   
>>   #define lockdep_recursing(tsk)	((tsk)->lockdep_recursion)
>>   
>> @@ -407,6 +405,9 @@ extern int lock_is_held(const void *);
>>   extern int lockdep_is_held(const void *);
>>   #define lockdep_is_held_type(l, r)		(1)
>>   
>> +#define lockdep_assert(c)			do { } while (0)
>> +#define lockdep_assert_once(c)			do { } while (0)
>> +
>>   #define lockdep_assert_held(l)			do { (void)(l); } while (0)
>>   #define lockdep_assert_not_held(l)		do { (void)(l); } while (0)
>>   #define lockdep_assert_held_write(l)		do { (void)(l); } while (0)
>>
> 