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

[Rob Herring]

On Fri, Nov 29, 2019 at 8:33 AM Boris Brezillon
<boris.brezillon at collabora.com> wrote:
>
> On Fri, 29 Nov 2019 14:24:48 +0000
> Steven Price <steven.price at 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.

I'm not so sure there's a race. If there is, we still check madv value
when purging, so it would be harmless even if the state is
inconsistent.

> > > 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 at vger.kernel.org>
> > > Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> >
> > Reviewed-by: Steven Price <steven.price at 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);

This means we now hold the shrinker_lock while we take the pages_lock.
Is lockdep happy with this change? I suspect not given all the fun I
had getting lockdep happy.

Rob