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Date:   Thu, 29 Jul 2021 09:00:35 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        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,
        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
Message-ID: <YQJSE3TMRydDNhqT@phenom.ffwll.local>
Mail-Followup-To: Peter Zijlstra <peterz@infradead.org>,
        Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        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,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org
References: <20210722092929.244629-1-desmondcheongzx@gmail.com>
 <20210722092929.244629-2-desmondcheongzx@gmail.com>
 <YPlKkvelm/mcnCj0@phenom.ffwll.local>
 <YQAaIrNUXa6i2gxD@hirez.programming.kicks-ass.net>
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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
> 
> ---
> 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)
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch