linux-kernel - Re: [PATCH v18 08/26] drm/shmem-helper: Add and use lockless drm_gem_shmem_get

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Message-ID: <20231124122059.244250c7@collabora.com>
Date:   Fri, 24 Nov 2023 12:20:59 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Maxime Ripard <mripard@...nel.org>
Cc:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v18 08/26] drm/shmem-helper: Add and use lockless
 drm_gem_shmem_get_pages()

On Fri, 24 Nov 2023 11:47:57 +0100
Maxime Ripard <mripard@...nel.org> wrote:

> On Mon, Oct 30, 2023 at 02:01:47AM +0300, Dmitry Osipenko wrote:
> > Add lockless drm_gem_shmem_get_pages() helper that skips taking reservation
> > lock if pages_use_count is non-zero, leveraging from atomicity of the
> > refcount_t. Make drm_gem_shmem_mmap() to utilize the new helper.
> > 
> > Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> > Suggested-by: Boris Brezillon <boris.brezillon@...labora.com>
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> > ---
> >  drivers/gpu/drm/drm_gem_shmem_helper.c | 19 +++++++++++++++----
> >  1 file changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index 6e02643ed87e..41b749bedb11 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -226,6 +226,20 @@ void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
> >  }
> >  EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
> >  
> > +static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
> > +{
> > +	int ret;
> > +
> > +	if (refcount_inc_not_zero(&shmem->pages_use_count))
> > +		return 0;
> > +
> > +	dma_resv_lock(shmem->base.resv, NULL);
> > +	ret = drm_gem_shmem_get_pages_locked(shmem);
> > +	dma_resv_unlock(shmem->base.resv);
> > +
> > +	return ret;
> > +}
> > +  
> 
> Wait, so the locked suffix is to indicate that we need to take the lock
> before we call it? I think that's the opposite to all(?) the naming
> convention we have

If you grep for "_locked(" and "_unlocked(" in the DRM sub-tree, you'll
see it's actually mixed, with maybe a few more helpers suffixed
_locked() than we have suffixed with _unlocked().

> 
> Especially since the function name doesn't describe what the function
> does anymore, but the context in which to call it.

Well, that's the same for "_unlocked", and we do have to pick one of
the _locked/_unlocked pattern if we want to expose both flavors.

> I'm sure if I was to
> use it, I would have gotten it wrong, or at the very least been very
> confused about it.

I personally find both equally confusing tbh, but we do have cases
where we need to expose the exact same functionality without the extra
locking. I do have a slight preference for _locked though, because it's
two characters shorter ;-).