Re: [PATCH v3 4/5] drm/i915/gem: Fix same-driver-another-instance dma-buf export

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 28, 2021 at 07:45:31PM +0000, Ruhl, Michael J wrote:
> >-----Original Message-----
> >From: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> >Sent: Monday, June 28, 2021 10:46 AM
> >To: intel-gfx@lists.freedesktop.org; dri-devel@lists.freedesktop.org
> >Cc: Auld, Matthew <matthew.auld@intel.com>;
> >maarten.lankhorst@linux.intel.com; Thomas Hellström
> ><thomas.hellstrom@linux.intel.com>; Ruhl; Ruhl, Michael J
> ><michael.j.ruhl@intel.com>
> >Subject: [PATCH v3 4/5] drm/i915/gem: Fix same-driver-another-instance
> >dma-buf export
> >
> >If our exported dma-bufs are imported by another instance of our driver,
> >that instance will typically have the imported dma-bufs locked during
> >map_attachment(). But the exporter also locks the same reservation
> >object in the map_dma_buf() callback, which leads to recursive locking.
> >
> >Add a live selftest to catch this case, and as a workaround until
> >we fully support dynamic import and export, declare the exporter dynamic
> >by providing NOP pin() and unpin() functions. This means our map_dma_buf()
> >callback will *always* get called locked, and by pinning unconditionally
> >in i915_gem_map_dma_buf() we make sure we don't need to use the
> >move_notify() functionality which is not yet implemented.
> 
> An interesting abuse of the interface, but it seems reasonable (for now) to me.

Hm I'm not sure this is the best interface abuse, since if we combine this
with amdgpu it goes boom. Also I thought the dynamic stuff is optional (or
is that only for the importer).

What I discussed a bit with Maarten on irc is to essentially emulate the
rules of what a dynamic exporter would end up with with a non-dynamic
importer: pin/unpin the buffer at attach/detach time. We could fake this
in our attach/detach callbacks.

At least I don't think it's the locking changes that saves us here, but
the caching of the sgt list in attach/detach. As long as we hand-roll that
we should be fine. So hand-rolling that feels like the best option to make
sure we're not making this worse, as long as we haven't fully validated
the true dynamic importer _and_ exporter case.

Cheers, Daniel

> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
> 
> Mike
> 
> >Reported-by: Ruhl, Michael J <michael.j.ruhl@intel.com>
> >Cc: Ruhl, Michael J <michael.j.ruhl@intel.com>
> >Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> >---
> > drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c    | 31 ++++++-
> > .../drm/i915/gem/selftests/i915_gem_dmabuf.c  | 81
> >++++++++++++++++++-
> > 2 files changed, 108 insertions(+), 4 deletions(-)
> >
> >diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> >b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> >index 616c3a2f1baf..1d1eeb167d28 100644
> >--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> >+++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> >@@ -12,6 +12,8 @@
> > #include "i915_gem_object.h"
> > #include "i915_scatterlist.h"
> >
> >+I915_SELFTEST_DECLARE(static bool force_different_devices;)
> >+
> > static struct drm_i915_gem_object *dma_buf_to_obj(struct dma_buf *buf)
> > {
> > 	return to_intel_bo(buf->priv);
> >@@ -25,7 +27,9 @@ static struct sg_table *i915_gem_map_dma_buf(struct
> >dma_buf_attachment *attachme
> > 	struct scatterlist *src, *dst;
> > 	int ret, i;
> >
> >-	ret = i915_gem_object_pin_pages_unlocked(obj);
> >+	assert_object_held(obj);
> >+
> >+	ret = i915_gem_object_pin_pages(obj);
> > 	if (ret)
> > 		goto err;
> >
> >@@ -168,6 +172,26 @@ static int i915_gem_end_cpu_access(struct dma_buf
> >*dma_buf, enum dma_data_direct
> > 	return err;
> > }
> >
> >+/*
> >+ * As a workaround until we fully support dynamic import and export,
> >+ * declare the exporter dynamic by providing NOP pin() and unpin()
> >functions.
> >+ * This means our i915_gem_map_dma_buf() callback will *always* get
> >called
> >+ * locked, and by pinning unconditionally in i915_gem_map_dma_buf() we
> >make
> >+ * sure we don't need to use the move_notify() functionality which is
> >+ * not yet implemented. Typically for the same-driver-another-instance case,
> >+ * i915_gem_map_dma_buf() will be called at importer attach time and the
> >+ * mapped sg_list will be cached by the dma-buf core for the
> >+ * duration of the attachment.
> >+ */
> >+static int i915_gem_dmabuf_pin(struct dma_buf_attachment *attach)
> >+{
> >+	return 0;
> >+}
> >+
> >+static void i915_gem_dmabuf_unpin(struct dma_buf_attachment *attach)
> >+{
> >+}
> >+
> > static const struct dma_buf_ops i915_dmabuf_ops =  {
> > 	.map_dma_buf = i915_gem_map_dma_buf,
> > 	.unmap_dma_buf = i915_gem_unmap_dma_buf,
> >@@ -177,6 +201,8 @@ static const struct dma_buf_ops i915_dmabuf_ops =  {
> > 	.vunmap = i915_gem_dmabuf_vunmap,
> > 	.begin_cpu_access = i915_gem_begin_cpu_access,
> > 	.end_cpu_access = i915_gem_end_cpu_access,
> >+	.pin = i915_gem_dmabuf_pin,
> >+	.unpin = i915_gem_dmabuf_unpin,
> > };
> >
> > struct dma_buf *i915_gem_prime_export(struct drm_gem_object
> >*gem_obj, int flags)
> >@@ -241,7 +267,8 @@ struct drm_gem_object
> >*i915_gem_prime_import(struct drm_device *dev,
> > 	if (dma_buf->ops == &i915_dmabuf_ops) {
> > 		obj = dma_buf_to_obj(dma_buf);
> > 		/* is it from our device? */
> >-		if (obj->base.dev == dev) {
> >+		if (obj->base.dev == dev &&
> >+		    !I915_SELFTEST_ONLY(force_different_devices)) {
> > 			/*
> > 			 * Importing dmabuf exported from out own gem
> >increases
> > 			 * refcount on gem itself instead of f_count of
> >dmabuf.
> >diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> >b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> >index dd74bc09ec88..24735d6c12a2 100644
> >--- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> >+++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_dmabuf.c
> >@@ -35,7 +35,7 @@ static int igt_dmabuf_export(void *arg)
> > static int igt_dmabuf_import_self(void *arg)
> > {
> > 	struct drm_i915_private *i915 = arg;
> >-	struct drm_i915_gem_object *obj;
> >+	struct drm_i915_gem_object *obj, *import_obj;
> > 	struct drm_gem_object *import;
> > 	struct dma_buf *dmabuf;
> > 	int err;
> >@@ -65,14 +65,90 @@ static int igt_dmabuf_import_self(void *arg)
> > 		err = -EINVAL;
> > 		goto out_import;
> > 	}
> >+	import_obj = to_intel_bo(import);
> >+
> >+	i915_gem_object_lock(import_obj, NULL);
> >+	err = ____i915_gem_object_get_pages(import_obj);
> >+	i915_gem_object_unlock(import_obj);
> >+	if (err) {
> >+		pr_err("Same object dma-buf get_pages failed!\n");
> >+		goto out_import;
> >+	}
> >
> > 	err = 0;
> > out_import:
> >-	i915_gem_object_put(to_intel_bo(import));
> >+	i915_gem_object_put(import_obj);
> >+out_dmabuf:
> >+	dma_buf_put(dmabuf);
> >+out:
> >+	i915_gem_object_put(obj);
> >+	return err;
> >+}
> >+
> >+static int igt_dmabuf_import_same_driver(void *arg)
> >+{
> >+	struct drm_i915_private *i915 = arg;
> >+	struct drm_i915_gem_object *obj, *import_obj;
> >+	struct drm_gem_object *import;
> >+	struct dma_buf *dmabuf;
> >+	int err;
> >+
> >+	force_different_devices = true;
> >+	obj = i915_gem_object_create_shmem(i915, PAGE_SIZE);
> >+	if (IS_ERR(obj))
> >+		goto out_ret;
> >+
> >+	dmabuf = i915_gem_prime_export(&obj->base, 0);
> >+	if (IS_ERR(dmabuf)) {
> >+		pr_err("i915_gem_prime_export failed with err=%d\n",
> >+		       (int)PTR_ERR(dmabuf));
> >+		err = PTR_ERR(dmabuf);
> >+		goto out;
> >+	}
> >+
> >+	import = i915_gem_prime_import(&i915->drm, dmabuf);
> >+	if (IS_ERR(import)) {
> >+		pr_err("i915_gem_prime_import failed with err=%d\n",
> >+		       (int)PTR_ERR(import));
> >+		err = PTR_ERR(import);
> >+		goto out_dmabuf;
> >+	}
> >+
> >+	if (import == &obj->base) {
> >+		pr_err("i915_gem_prime_import reused gem object!\n");
> >+		err = -EINVAL;
> >+		goto out_import;
> >+	}
> >+
> >+	import_obj = to_intel_bo(import);
> >+
> >+	i915_gem_object_lock(import_obj, NULL);
> >+	err = ____i915_gem_object_get_pages(import_obj);
> >+	if (err) {
> >+		pr_err("Different objects dma-buf get_pages failed!\n");
> >+		i915_gem_object_unlock(import_obj);
> >+		goto out_import;
> >+	}
> >+
> >+	/*
> >+	 * If the exported object is not in system memory, something
> >+	 * weird is going on. TODO: When p2p is supported, this is no
> >+	 * longer considered weird.
> >+	 */
> >+	if (obj->mm.region != i915->mm.regions[INTEL_REGION_SMEM]) {
> >+		pr_err("Exported dma-buf is not in system memory\n");
> >+		err = -EINVAL;
> >+	}
> >+	i915_gem_object_unlock(import_obj);
> >+
> >+out_import:
> >+	i915_gem_object_put(import_obj);
> > out_dmabuf:
> > 	dma_buf_put(dmabuf);
> > out:
> > 	i915_gem_object_put(obj);
> >+out_ret:
> >+	force_different_devices = false;
> > 	return err;
> > }
> >
> >@@ -286,6 +362,7 @@ int i915_gem_dmabuf_live_selftests(struct
> >drm_i915_private *i915)
> > {
> > 	static const struct i915_subtest tests[] = {
> > 		SUBTEST(igt_dmabuf_export),
> >+		SUBTEST(igt_dmabuf_import_same_driver),
> > 	};
> >
> > 	return i915_subtests(tests, i915);
> >--
> >2.31.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch