linux-kernel - Re: [PATCH] drm/panthor: Lock XArray when getting entries for heap and VM

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Message-ID: <20d75e2c-c5a5-48c3-ac99-a9e15f19b872@arm.com>
Date: Wed, 6 Nov 2024 13:17:29 +0000
From: Steven Price <steven.price@....com>
To: Liviu Dudau <liviu.dudau@....com>,
 Boris Brezillon <boris.brezillon@...labora.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] drm/panthor: Lock XArray when getting entries for heap
 and VM

On 06/11/2024 12:07, Liviu Dudau wrote:
> Similar to cac075706f29 ("drm/panthor: Fix race when converting
> group handle to group object") we need to use the XArray's internal
> locking when retrieving a pointer from there for heap and vm.
> 
> Reported-by: Jann Horn <jannh@...gle.com>
> Cc: Boris Brezillon <boris.brezillon@...labora.com>
> Cc: Steven Price <steven.price@....com>
> Signed-off-by: Liviu Dudau <liviu.dudau@....com>
> ---
>  drivers/gpu/drm/panthor/panthor_heap.c | 15 +++++++++++++--
>  drivers/gpu/drm/panthor/panthor_mmu.c  |  2 ++
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_heap.c b/drivers/gpu/drm/panthor/panthor_heap.c
> index 3796a9eb22af2..fe0bcb6837f74 100644
> --- a/drivers/gpu/drm/panthor/panthor_heap.c
> +++ b/drivers/gpu/drm/panthor/panthor_heap.c
> @@ -351,6 +351,17 @@ int panthor_heap_create(struct panthor_heap_pool *pool,
>  	return ret;
>  }
>  
> +static struct panthor_heap *panthor_heap_from_id(struct pathor_heap_pool *pool, u32 id)
> +{
> +	struct panthor_heap *heap;
> +
> +	xa_lock(&pool->xa);
> +	heap = xa_load(&pool->xa, id);
> +	xa_unlock(&pool->va);
> +
> +	return heap;
> +}

This locking doesn't actually achieve anything - XArray already deals
with the concurrency (with RCU), and if we're doing nothing more than an
xa_load() then we don't need (extra) locking (unless using the __
prefixed functions).

And, as Boris has pointed out, pool->lock is held. As you mention in
your email the missing bit might be panthor_heap_pool_release() - if
it's not holding a lock then the heap could be freed immediately after
panthor_heap_from_id() returns (even with the above change).

Steve

> +
>  /**
>   * panthor_heap_return_chunk() - Return an unused heap chunk
>   * @pool: The pool this heap belongs to.
> @@ -375,7 +386,7 @@ int panthor_heap_return_chunk(struct panthor_heap_pool *pool,
>  		return -EINVAL;
>  
>  	down_read(&pool->lock);
> -	heap = xa_load(&pool->xa, heap_id);
> +	heap = panthor_heap_from_id(pool, heap_id);
>  	if (!heap) {
>  		ret = -EINVAL;
>  		goto out_unlock;
> @@ -438,7 +449,7 @@ int panthor_heap_grow(struct panthor_heap_pool *pool,
>  		return -EINVAL;
>  
>  	down_read(&pool->lock);
> -	heap = xa_load(&pool->xa, heap_id);
> +	heap = panthor_heap_from_id(pool, heap_id);
>  	if (!heap) {
>  		ret = -EINVAL;
>  		goto out_unlock;
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 8ca85526491e6..8b5cda9d21768 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -1580,7 +1580,9 @@ panthor_vm_pool_get_vm(struct panthor_vm_pool *pool, u32 handle)
>  {
>  	struct panthor_vm *vm;
>  
> +	xa_lock(&pool->xa);
>  	vm = panthor_vm_get(xa_load(&pool->xa, handle));
> +	xa_unlock(&pool->va);
>  
>  	return vm;
>  }