linux-kernel - Re: [PATCH v1] drm/shmem-helper: Fix unsetting shmem vaddr while vmap refcount

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Message-ID: <20250403163047.63929f63@collabora.com>
Date: Thu, 3 Apr 2025 16:30:47 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: Thomas Zimmermann <tzimmermann@...e.de>, Lucas De Marchi
 <lucas.demarchi@...el.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v1] drm/shmem-helper: Fix unsetting shmem vaddr while
 vmap refcount > 0

On Thu,  3 Apr 2025 17:26:33 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> We switched to use refcount_t for vmaps and missed to change the vunmap
> code to properly unset the vmap pointer, which is now cleared while vmap's
> refcount > 0. Clear the cached vmap pointer only when refcounting drops to
> zero to fix the bug.
> 
> Fixes: e1fc39a92332 ("drm/shmem-helper: Use refcount_t for vmap_use_count")
> Reported-by: Lucas De Marchi <lucas.demarchi@...el.com>
> Closes: https://lore.kernel.org/dri-devel/20250403105053.788b0f6e@collabora.com/T/#m3dca6d81bedc8d6146a56b82694624fbc6fa4c96
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>

> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 2d924d547a51..aa43265f4f4f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -415,11 +415,11 @@ void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>  
>  		if (refcount_dec_and_test(&shmem->vmap_use_count)) {
>  			vunmap(shmem->vaddr);
> +			shmem->vaddr = NULL;
> +
>  			drm_gem_shmem_unpin_locked(shmem);
>  		}
>  	}
> -
> -	shmem->vaddr = NULL;
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_vunmap_locked);
>