linux-kernel - Re: [PATCH 2/2] drm: add lockdep assert to drm_is_current_master

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Message-ID: <35518f4b-5e4a-b284-1f86-5cba64941211@gmail.com>
Date:   Fri, 30 Jul 2021 16:06:44 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH 2/2] drm: add lockdep assert to
 drm_is_current_master_locked

On 30/7/21 2:08 pm, Boqun Feng wrote:
> On Fri, Jul 30, 2021 at 12:15:15PM +0800, Desmond Cheong Zhi Xi wrote:
>> In drm_is_current_master_locked, accessing drm_file.master should be
>> protected by either drm_file.master_lookup_lock or
>> drm_device.master_mutex. This was previously awkward to assert with
>> lockdep.
>>
>> Following patch ("locking/lockdep: Provide lockdep_assert{,_once}()
>> helpers"), this assertion is now convenient so we add it in.
>>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
>> ---
>>   drivers/gpu/drm/drm_auth.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index 9c24b8cc8e36..6f4d7ff23c80 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -63,9 +63,9 @@
>>   
>>   static bool drm_is_current_master_locked(struct drm_file *fpriv)
>>   {
>> -	/* Either drm_device.master_mutex or drm_file.master_lookup_lock
>> -	 * should be held here.
>> -	 */
>> +	lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
>> +			    lockdep_is_held(&fpriv->minor->dev->master_mutex));
>> +
> 
> I think it's better to also add the lockdep_assert() of & (i.e. both
> held) in the updater side, and have comments pointing to each other.
> 
> Is it convenient to do in this patchset? If the updater side doesn't
> need to put the lockdep_assert() (maybe the lock acquire code and the
> update code are in the same function), it's still better to add some

Thanks for the feedback, Boqun.

Yeah, I think the updater side maybe doesn't need new lockdep_assert()
because what currently happens is either

	lockdep_assert_held_once(&dev->master_mutex);
	/* 6 lines of prep */
	spin_lock(&fpriv->master_lookup_lock);
	fpriv->master = new_value;
or
	mutex_lock(&dev->master_mutex);
	/* 3 lines of checks */
		spin_lock(&file_priv->master_lookup_lock);
		file_priv->master = new_value;

> comments like:
> 
> 	/*
> 	 * To update drm_file.master, both drm_file.master_lookup_lock
> 	 * and drm_device.master_mutex are needed, therefore holding
> 	 * either of them is safe and enough for the read side.
> 	 */
> 
> Just feel it's better to explain the lock design either in the
> lockdep_assert() or comments.
> 

But clarifying the lock design in the documentation sounds like a really
good idea.

Probably a good place for this would be in the kerneldoc where we also
explain the lifetime rules and usage of the pointer outside drm_auth.c:

diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 726cfe0ff5f5..a3acb7ac3550 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -233,6 +233,10 @@ struct drm_file {
  	 * this only matches &drm_device.master if the master is the currently
  	 * active one.
  	 *
+	 * To update @master, both &drm_device.master_mutex and
+	 * @master_lookup_lock need to be held, therefore holding either of
+	 * them is safe and enough for the read side.
+	 *
  	 * When dereferencing this pointer, either hold struct
  	 * &drm_device.master_mutex for the duration of the pointer's use, or
  	 * use drm_file_get_master() if struct &drm_device.master_mutex is not

Best wishes,
Desmond

> Regards,
> Boqun
> 
>>   	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
>>   }
>>   
>> -- 
>> 2.25.1
>>