Re: [PATCH v4 6/6] drm/shmem-helper: Switch to reservation lock

Boris Brezillon
2023-06-26T09:58:01Z

On Mon, 26 Jun 2023 11:40:14 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Hi Dmitry,
>
> On Tue, 30 May 2023 01:39:35 +0300
> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
>
> > Replace all drm-shmem locks with a GEM reservation lock. This makes locks
> > consistent with dma-buf locking convention where importers are responsible
> > for holding reservation lock for all operations performed over dma-bufs,
> > preventing deadlock between dma-buf importers and exporters.
>
> I've rebased some of my work on drm-misc-next this morning and noticed
> that the drm_gem_shmem_get_pages() I was using to pin pages no longer
> exists, so I ended looking at this patch to check what I should use
> instead, and I have a few questions/comments.
>
> >
> > Suggested-by: Daniel Vetter <daniel@ffwll.ch>
> > Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
> > Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> > ---
> > drivers/gpu/drm/drm_gem_shmem_helper.c | 210 ++++++++----------
> > drivers/gpu/drm/lima/lima_gem.c | 8 +-
> > drivers/gpu/drm/panfrost/panfrost_drv.c | 7 +-
> > .../gpu/drm/panfrost/panfrost_gem_shrinker.c | 6 +-
> > drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +-
> > include/drm/drm_gem_shmem_helper.h | 14 +-
> > 6 files changed, 116 insertions(+), 148 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index 4ea6507a77e5..a783d2245599 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -88,8 +88,6 @@ __drm_gem_shmem_create(struct drm_device *dev, size_t size, bool private)
> > if (ret)
> > goto err_release;
> >
> > - mutex_init(&shmem->pages_lock);
> > - mutex_init(&shmem->vmap_lock);
> > INIT_LIST_HEAD(&shmem->madv_list);
> >
> > if (!private) {
> > @@ -141,11 +139,13 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> > {
> > struct drm_gem_object *obj = &shmem->base;
> >
> > - drm_WARN_ON(obj->dev, shmem->vmap_use_count);
> > -
> > if (obj->import_attach) {
> > drm_prime_gem_destroy(obj, shmem->sgt);
> > } else {
> > + dma_resv_lock(shmem->base.resv, NULL);
> > +
> > + drm_WARN_ON(obj->dev, shmem->vmap_use_count);
> > +
> > if (shmem->sgt) {
> > dma_unmap_sgtable(obj->dev->dev, shmem->sgt,
> > DMA_BIDIRECTIONAL, 0);
> > @@ -154,22 +154,24 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> > }
> > if (shmem->pages)
> > drm_gem_shmem_put_pages(shmem);
> > - }
> >
> > - drm_WARN_ON(obj->dev, shmem->pages_use_count);
> > + drm_WARN_ON(obj->dev, shmem->pages_use_count);
> > +
> > + dma_resv_unlock(shmem->base.resv);
> > + }
> >
> > drm_gem_object_release(obj);
> > - mutex_destroy(&shmem->pages_lock);
> > - mutex_destroy(&shmem->vmap_lock);
> > kfree(shmem);
> > }
> > EXPORT_SYMBOL_GPL(drm_gem_shmem_free);
> >
> > -static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
> > +static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
>
> I find this name change confusing, because the function requires the
> GEM resv lock to be held, and the _locked suffix was making it pretty
> clear.
>
> > {
> > struct drm_gem_object *obj = &shmem->base;
> > struct page **pages;
> >
> > + dma_resv_assert_held(shmem->base.resv);
> > +
> > if (shmem->pages_use_count++ > 0)
> > return 0;
> >
> > @@ -197,35 +199,16 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
> > }
> >
> > /*
> > - * drm_gem_shmem_get_pages - Allocate backing pages for a shmem GEM object
> > + * drm_gem_shmem_put_pages - Decrease use count on the backing pages for a shmem GEM object
> > * @shmem: shmem GEM object
> > *
> > - * This function makes sure that backing pages exists for the shmem GEM object
> > - * and increases the use count.
> > - *
> > - * Returns:
> > - * 0 on success or a negative error code on failure.
> > + * This function decreases the use count and puts the backing pages when use drops to zero.
> > */
> > -int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
> > +void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
>
> Same comment about the name change. That's even more confusing since
> this function was previously taking care of the locking. Also not sure
> why you'd want to expose this _put() helper when the _get() helper is
> private.
>
> > {
> > struct drm_gem_object *obj = &shmem->base;
> > - int ret;
> >
> > - drm_WARN_ON(obj->dev, obj->import_attach);
> > -
> > - ret = mutex_lock_interruptible(&shmem->pages_lock);
> > - if (ret)
> > - return ret;
> > - ret = drm_gem_shmem_get_pages_locked(shmem);
> > - mutex_unlock(&shmem->pages_lock);
> > -
> > - return ret;
> > -}
> > -EXPORT_SYMBOL(drm_gem_shmem_get_pages);
> > -
> > -static void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
> > -{
> > - struct drm_gem_object *obj = &shmem->base;
> > + dma_resv_assert_held(shmem->base.resv);
> >
> > if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
> > return;
> > @@ -243,20 +226,25 @@ static void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
> > shmem->pages_mark_accessed_on_put);
> > shmem->pages = NULL;
> > }
> > +EXPORT_SYMBOL(drm_gem_shmem_put_pages);
> >
> > -/*
> > - * drm_gem_shmem_put_pages - Decrease use count on the backing pages for a shmem GEM object
> > - * @shmem: shmem GEM object
> > - *
> > - * This function decreases the use count and puts the backing pages when use drops to zero.
> > - */
> > -void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
> > +static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
> > {
> > - mutex_lock(&shmem->pages_lock);
> > - drm_gem_shmem_put_pages_locked(shmem);
> > - mutex_unlock(&shmem->pages_lock);
> > + int ret;
> > +
> > + dma_resv_assert_held(shmem->base.resv);
> > +
> > + ret = drm_gem_shmem_get_pages(shmem);
> > +
> > + return ret;
> > +}
> > +
> > +static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
> > +{
> > + dma_resv_assert_held(shmem->base.resv);
> > +
> > + drm_gem_shmem_put_pages(shmem);
> > }
> > -EXPORT_SYMBOL(drm_gem_shmem_put_pages);
> >
> > /**
> > * drm_gem_shmem_pin - Pin backing pages for a shmem GEM object
> > @@ -271,10 +259,17 @@ EXPORT_SYMBOL(drm_gem_shmem_put_pages);
> > int drm_gem_shmem_pin(struct drm_gem_shmem_object *shmem)
> > {
> > struct drm_gem_object *obj = &shmem->base;
> > + int ret;
> >
> > drm_WARN_ON(obj->dev, obj->import_attach);
> >
> > - return drm_gem_shmem_get_pages(shmem);
> > + ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
> > + if (ret)
> > + return ret;
>
> I think here is the major problem I have with this patch: you've made
> drm_gem_shmem_{get_pages,pin}() private, which forces me to call
> drm_gem_shmem_pin() in a path where I already acquired the resv lock
> (using the drm_exec infra proposed by Christian). That would
> probably work if you were letting ret == -EALREADY go through, but I'm
> wondering if it wouldn't be preferable to expose
> drm_gem_shmem_pin_locked().
>
> > + ret = drm_gem_shmem_pin_locked(shmem);
> > + dma_resv_unlock(shmem->base.resv);
> > +
> > + return ret;
> > }
> > EXPORT_SYMBOL(drm_gem_shmem_pin);
> >
> > @@ -291,12 +286,29 @@ void drm_gem_shmem_unpin(struct drm_gem_shmem_object *shmem)
> >
> > drm_WARN_ON(obj->dev, obj->import_attach);
> >
> > - drm_gem_shmem_put_pages(shmem);
> > + dma_resv_lock(shmem->base.resv, NULL);
> > + drm_gem_shmem_unpin_locked(shmem);
> > + dma_resv_unlock(shmem->base.resv);
> > }
> > EXPORT_SYMBOL(drm_gem_shmem_unpin);
>
> If we want to be consistent, let's just expose drm_gem_shmem_unpin()
> and drm_gem_shmem_pin() and keep drm_gem_shmem_{get,put}_pages()
> private, or even better, rename them drm_gem_shmem_{pin,unpin}_locked()
> insert of having drm_gem_shmem_{pin,unpin}_locked() wrappers that just
> forward the call to drm_gem_shmem_{get,put}_pages().
>
> >
> > -static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
> > - struct iosys_map *map)
> > +/*
> > + * drm_gem_shmem_vmap - Create a virtual mapping for a shmem GEM object
> > + * @shmem: shmem GEM object
> > + * @map: Returns the kernel virtual address of the SHMEM GEM object's backing
> > + * store.
> > + *
> > + * This function makes sure that a contiguous kernel virtual address mapping
> > + * exists for the buffer backing the shmem GEM object. It hides the differences
> > + * between dma-buf imported and natively allocated objects.
> > + *
> > + * Acquired mappings should be cleaned up by calling drm_gem_shmem_vunmap().
> > + *
> > + * Returns:
> > + * 0 on success or a negative error code on failure.
> > + */
> > +int drm_gem_shmem_vmap(struct drm_gem_shmem_object *shmem,
> > + struct iosys_map *map)
>
> Same problem with this renaming: it's confusing because this function
> was previously taking care of the locking, and it's no longer the case.
> That's actually true for other public functions your patching, but I
> won't go over all of them.

vmap() is less of a problem, because we're not supposed to call
drm_gem_shmem_vmap() directly, but rather go through drm_gem_vmap().
This should probably be clarified in the doc though, with this sort of
disclaimer: "don't use this function to map stuff, only use it to
implement drm_gem_object_funcs::vmap()."

Also noticed that the drm_gem API has the _locked pattern reversed,
with a few drm_gem_xxx_unlocked() helper that take the lock and call
the drm_gem_xxx() function. Don't have a strong opinion on whether this
is better than the xxx_locked() and xxx() pattern or not, but the fact
things are inconsistent across the API (drm_gem_pin() is letting the
backend take the lock before pinning pages, when drm_gem_vmap() is
not) is super confusing.