Re: [PATCH 2/8] drm/panfrost: Fix a race in panfrost_ioctl_madvise()

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

On Fri, 29 Nov 2019 14:24:48 +0000
Steven Price <steven.price@arm.com> wrote:

> On 29/11/2019 13:59, Boris Brezillon wrote:
> > If 2 threads change the MADVISE property of the same BO in parallel we
> > might end up with an shmem->madv value that's inconsistent with the
> > presence of the BO in the shrinker list.  
> 
> I'm a bit worried from the point of view of user space sanity that you
> observed this - but clearly the kernel should be robust!

It's not something I observed, just found the race by inspecting the
code, and I thought it was worth fixing it.

> 
> > 
> > The easiest solution to fix that is to protect the
> > drm_gem_shmem_madvise() call with the shrinker lock.
> > 
> > Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>  
> 
> Reviewed-by: Steven Price <steven.price@arm.com>

Thanks.

> 
> > ---
> >  drivers/gpu/drm/panfrost/panfrost_drv.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> > index f21bc8a7ee3a..efc0a24d1f4c 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> > @@ -347,20 +347,19 @@ static int panfrost_ioctl_madvise(struct drm_device *dev, void *data,
> >  		return -ENOENT;
> >  	}
> >  
> > +	mutex_lock(&pfdev->shrinker_lock);
> >  	args->retained = drm_gem_shmem_madvise(gem_obj, args->madv);
> >  
> >  	if (args->retained) {
> >  		struct panfrost_gem_object *bo = to_panfrost_bo(gem_obj);
> >  
> > -		mutex_lock(&pfdev->shrinker_lock);
> > -
> >  		if (args->madv == PANFROST_MADV_DONTNEED)
> > -			list_add_tail(&bo->base.madv_list, &pfdev->shrinker_list);
> > +			list_add_tail(&bo->base.madv_list,
> > +				      &pfdev->shrinker_list);
> >  		else if (args->madv == PANFROST_MADV_WILLNEED)
> >  			list_del_init(&bo->base.madv_list);
> > -
> > -		mutex_unlock(&pfdev->shrinker_lock);
> >  	}
> > +	mutex_unlock(&pfdev->shrinker_lock);
> >  
> >  	drm_gem_object_put_unlocked(gem_obj);
> >  	return 0;
> >   
>