Date:   Fri, 17 Feb 2023 13:18:50 -0500
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Danilo Krummrich <dakr@redhat.com>
Cc:     airlied@gmail.com, daniel@ffwll.ch, tzimmermann@suse.de,
        mripard@kernel.org, corbet@lwn.net, christian.koenig@amd.com,
        bskeggs@redhat.com, matthew.brost@intel.com,
        boris.brezillon@collabora.com, alexdeucher@gmail.com,
        ogabbay@kernel.org, bagasdotme@gmail.com, willy@infradead.org,
        jason@jlekstrand.net, dri-devel@lists.freedesktop.org,
        nouveau@lists.freedesktop.org, linux-doc@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH drm-next v2 04/16] maple_tree: add flag MT_FLAGS_LOCK_NONE
Message-ID: <20230217181850.mq7ua6rksl3ezwoi@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        Danilo Krummrich <dakr@redhat.com>, airlied@gmail.com,
        daniel@ffwll.ch, tzimmermann@suse.de, mripard@kernel.org,
        corbet@lwn.net, christian.koenig@amd.com, bskeggs@redhat.com,
        matthew.brost@intel.com, boris.brezillon@collabora.com,
        alexdeucher@gmail.com, ogabbay@kernel.org, bagasdotme@gmail.com,
        willy@infradead.org, jason@jlekstrand.net,
        dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230217134422.14116-1-dakr@redhat.com>
 <20230217134422.14116-5-dakr@redhat.com>
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* Danilo Krummrich <dakr@redhat.com> [230217 08:44]:
> Generic components making use of the maple tree (such as the
> DRM GPUVA Manager) delegate the responsibility of ensuring mutual
> exclusion to their users.
> 
> While such components could inherit the concept of an external lock,
> some users might just serialize the access to the component and hence to
> the internal maple tree.
> 
> In order to allow such use cases, add a new flag MT_FLAGS_LOCK_NONE to
> indicate not to do any internal lockdep checks.
> 
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  include/linux/maple_tree.h | 20 +++++++++++++++-----
>  lib/maple_tree.c           |  7 ++++---
>  2 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/maple_tree.h b/include/linux/maple_tree.h
> index ca04c900e51a..f795e5def8d0 100644
> --- a/include/linux/maple_tree.h
> +++ b/include/linux/maple_tree.h
> @@ -170,10 +170,11 @@ enum maple_type {
>  #define MT_FLAGS_USE_RCU	0x02
>  #define MT_FLAGS_HEIGHT_OFFSET	0x02
>  #define MT_FLAGS_HEIGHT_MASK	0x7C
> -#define MT_FLAGS_LOCK_MASK	0x300
> +#define MT_FLAGS_LOCK_MASK	0x700
>  #define MT_FLAGS_LOCK_IRQ	0x100
>  #define MT_FLAGS_LOCK_BH	0x200
>  #define MT_FLAGS_LOCK_EXTERN	0x300
> +#define MT_FLAGS_LOCK_NONE	0x400

Please add this to the documentation above the flags as well.  We should
probably add enough context so that users don't just set this and then
use multiple writers.

>  
>  #define MAPLE_HEIGHT_MAX	31
>  
> @@ -559,11 +560,16 @@ static inline void mas_set(struct ma_state *mas, unsigned long index)
>  	mas_set_range(mas, index, index);
>  }
>  
> -static inline bool mt_external_lock(const struct maple_tree *mt)
> +static inline bool mt_lock_external(const struct maple_tree *mt)
>  {
>  	return (mt->ma_flags & MT_FLAGS_LOCK_MASK) == MT_FLAGS_LOCK_EXTERN;
>  }
>  
> +static inline bool mt_lock_none(const struct maple_tree *mt)
> +{
> +	return (mt->ma_flags & MT_FLAGS_LOCK_MASK) == MT_FLAGS_LOCK_NONE;
> +}
> +
>  /**
>   * mt_init_flags() - Initialise an empty maple tree with flags.
>   * @mt: Maple Tree
> @@ -577,7 +583,7 @@ static inline bool mt_external_lock(const struct maple_tree *mt)
>  static inline void mt_init_flags(struct maple_tree *mt, unsigned int flags)
>  {
>  	mt->ma_flags = flags;
> -	if (!mt_external_lock(mt))
> +	if (!mt_lock_external(mt) && !mt_lock_none(mt))
>  		spin_lock_init(&mt->ma_lock);
>  	rcu_assign_pointer(mt->ma_root, NULL);
>  }
> @@ -612,9 +618,11 @@ static inline void mt_clear_in_rcu(struct maple_tree *mt)
>  	if (!mt_in_rcu(mt))
>  		return;
>  
> -	if (mt_external_lock(mt)) {
> +	if (mt_lock_external(mt)) {
>  		BUG_ON(!mt_lock_is_held(mt));
>  		mt->ma_flags &= ~MT_FLAGS_USE_RCU;
> +	} else if (mt_lock_none(mt)) {
> +		mt->ma_flags &= ~MT_FLAGS_USE_RCU;
>  	} else {
>  		mtree_lock(mt);
>  		mt->ma_flags &= ~MT_FLAGS_USE_RCU;
> @@ -631,9 +639,11 @@ static inline void mt_set_in_rcu(struct maple_tree *mt)
>  	if (mt_in_rcu(mt))
>  		return;
>  
> -	if (mt_external_lock(mt)) {
> +	if (mt_lock_external(mt)) {
>  		BUG_ON(!mt_lock_is_held(mt));
>  		mt->ma_flags |= MT_FLAGS_USE_RCU;
> +	} else if (mt_lock_none(mt)) {
> +		mt->ma_flags |= MT_FLAGS_USE_RCU;
>  	} else {
>  		mtree_lock(mt);
>  		mt->ma_flags |= MT_FLAGS_USE_RCU;
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 26e2045d3cda..f51c0fd4eaad 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -802,8 +802,8 @@ static inline void __rcu **ma_slots(struct maple_node *mn, enum maple_type mt)
>  
>  static inline bool mt_locked(const struct maple_tree *mt)
>  {
> -	return mt_external_lock(mt) ? mt_lock_is_held(mt) :
> -		lockdep_is_held(&mt->ma_lock);
> +	return mt_lock_external(mt) ? mt_lock_is_held(mt) :
> +		mt_lock_none(mt) ? true : lockdep_is_held(&mt->ma_lock);

It might be better to just make this two return statements for clarity.

>  }
>  
>  static inline void *mt_slot(const struct maple_tree *mt,
> @@ -6120,7 +6120,8 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
>  		return false;
>  	}
>  
> -	if (gfpflags_allow_blocking(gfp) && !mt_external_lock(mas->tree)) {
> +	if (gfpflags_allow_blocking(gfp) &&
> +	    !mt_lock_external(mas->tree) && !mt_lock_none(mas->tree)) {
>  		mtree_unlock(mas->tree);
>  		mas_alloc_nodes(mas, gfp);
>  		mtree_lock(mas->tree);
> -- 
> 2.39.1
>