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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Wed, 6 Nov 2024 13:17:29 +0000
Steven Price <steven.price@arm.com> wrote:

> 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@google.com>
> > Cc: Boris Brezillon <boris.brezillon@collabora.com>
> > Cc: Steven Price <steven.price@arm.com>
> > Signed-off-by: Liviu Dudau <liviu.dudau@arm.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).

Hm, if we call panthor_heap_from_id(), that means we have a heap pool to
pass, and incidentally, we're supposed to hold a ref on this pool. So I
don't really see how the heap pool can go away, unless someone messed
up with the refcounting in the meantime.

> 
> 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;
> >  }  
>