Re: [PATCH 1/2] drm/i915/gem: Correct the locking and pin pattern for dma-buf

Christian König <ckoenig.leichtzumerken@xxxxxxxxx> · Wed, 7 Jul 2021 08:42:20 +0200

Am 02.07.21 um 19:09 schrieb Daniel Vetter:
On Thu, Jul 1, 2021 at 4:24 PM Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx> wrote:
From: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>

If our exported dma-bufs are imported by another instance of our driver,
that instance will typically have the imported dma-bufs locked during
dma_buf_map_attachment(). But the exporter also locks the same reservation
object in the map_dma_buf() callback, which leads to recursive locking.

So taking the lock inside _pin_pages_unlocked() is incorrect.

Additionally, the current pinning code path is contrary to the defined
way that pinning should occur.

Remove the explicit pin/unpin from the map/umap functions and move them
to the attach/detach allowing correct locking to occur, and to match
the static dma-buf drm_prime pattern.

Add a live selftest to exercise both dynamic and non-dynamic
exports.

v2:
- Extend the selftest with a fake dynamic importer.
- Provide real pin and unpin callbacks to not abuse the interface.
v3: (ruhl)
- Remove the dynamic export support and move the pinning into the
   attach/detach path.

Reported-by: Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx>
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx>
CI splat is because I got the locking rules wrong, I thought
->attach/detach are called under the dma_resv_lock, because when we
used the old dma_buf->lock those calls where protected by that lock
under the same critical section as adding/removing from the list. But
we changed that in

f45f57cce584 ("dma-buf: stop using the dmabuf->lock so much v2")
15fd552d186c ("dma-buf: change DMA-buf locking convention v3")

Because keeping dma_resv_lock over ->attach/detach would go boom on
all the ttm drivers, which pin/unpin the buffer in there. Iow we need
the unlocked version there, but also having this split up is a bit
awkward and might be good to patch up so that it's atomic again. Would
mean updating a bunch of drivers. Christian, any thoughts?

Yeah, we already discussed that when we switched from dma_buf->lock to 
dma_resv->lock.

In general completely agree to do this and stopping using the 
dma_buf->lock was a first step towards this, but IIRC we postponed that 
switch till later.

Regards,
Christian.

PS: I'm currently on sick leave, so response might be delayed.


Mike, for now I'd just keep using the _unlocked variants and we should be fine.
-Daniel

---
  drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c    |  46 ++++++--
  .../drm/i915/gem/selftests/i915_gem_dmabuf.c  | 111 +++++++++++++++++-
  2 files changed, 143 insertions(+), 14 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..00338c8d3739 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);
@@ -25,15 +27,11 @@ 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);
-       if (ret)
-               goto err;
-
         /* Copy sg so that we make an independent mapping */
         st = kmalloc(sizeof(struct sg_table), GFP_KERNEL);
         if (st == NULL) {
                 ret = -ENOMEM;
-               goto err_unpin_pages;
+               goto err;
         }

         ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL);
@@ -58,8 +56,6 @@ static struct sg_table *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme
         sg_free_table(st);
  err_free:
         kfree(st);
-err_unpin_pages:
-       i915_gem_object_unpin_pages(obj);
  err:
         return ERR_PTR(ret);
  }
@@ -68,13 +64,9 @@ static void i915_gem_unmap_dma_buf(struct dma_buf_attachment *attachment,
                                    struct sg_table *sg,
                                    enum dma_data_direction dir)
  {
-       struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment->dmabuf);
-
         dma_unmap_sgtable(attachment->dev, sg, dir, DMA_ATTR_SKIP_CPU_SYNC);
         sg_free_table(sg);
         kfree(sg);
-
-       i915_gem_object_unpin_pages(obj);
  }

  static int i915_gem_dmabuf_vmap(struct dma_buf *dma_buf, struct dma_buf_map *map)
@@ -168,7 +160,32 @@ static int i915_gem_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direct
         return err;
  }

+/**
+ * i915_gem_dmabuf_attach - Do any extra attach work necessary
+ * @dmabuf: imported dma-buf
+ * @attach: new attach to do work on
+ *
+ */
+static int i915_gem_dmabuf_attach(struct dma_buf *dmabuf,
+                                 struct dma_buf_attachment *attach)
+{
+       struct drm_i915_gem_object *obj = dma_buf_to_obj(dmabuf);
+
+       assert_object_held(obj);
+       return i915_gem_object_pin_pages(obj);
+}
+
+static void i915_gem_dmabuf_detach(struct dma_buf *dmabuf,
+                                 struct dma_buf_attachment *attach)
+{
+       struct drm_i915_gem_object *obj = dma_buf_to_obj(dmabuf);
+
+       i915_gem_object_unpin_pages(obj);
+}
+
  static const struct dma_buf_ops i915_dmabuf_ops =  {
+       .attach = i915_gem_dmabuf_attach,
+       .detach = i915_gem_dmabuf_detach,
         .map_dma_buf = i915_gem_map_dma_buf,
         .unmap_dma_buf = i915_gem_unmap_dma_buf,
         .release = drm_gem_dmabuf_release,
@@ -204,6 +221,8 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
         struct sg_table *pages;
         unsigned int sg_page_sizes;

+       assert_object_held(obj);
+
         pages = dma_buf_map_attachment(obj->base.import_attach,
                                        DMA_BIDIRECTIONAL);
         if (IS_ERR(pages))
@@ -219,6 +238,8 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
  static void i915_gem_object_put_pages_dmabuf(struct drm_i915_gem_object *obj,
                                              struct sg_table *pages)
  {
+       assert_object_held(obj);
+
         dma_buf_unmap_attachment(obj->base.import_attach, pages,
                                  DMA_BIDIRECTIONAL);
  }
@@ -241,7 +262,8 @@ 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_differnt_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..10a113cc00a5 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 +65,120 @@ 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 const struct dma_buf_attach_ops igt_dmabuf_attach_ops = {
+       .move_notify = igt_dmabuf_move_notify,
+};
+
+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;
+       struct dma_buf_attachment *import_attach;
+       struct sg_table *st;
+       long timeout;
+       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);
+
+       /* Now try a fake dynamic importer */
+       import_attach = dma_buf_dynamic_attach(dmabuf, obj->base.dev->dev,
+                                              &igt_dmabuf_attach_ops,
+                                              NULL);
+       if (IS_ERR(import_attach))
+               goto out_import;
+
+       dma_resv_lock(dmabuf->resv, NULL);
+       st = dma_buf_map_attachment(import_attach, DMA_BIDIRECTIONAL);
+       dma_resv_unlock(dmabuf->resv);
+       if (IS_ERR(st))
+               goto out_detach;
+
+       timeout = dma_resv_wait_timeout(dmabuf->resv, false, true, 5 * HZ);
+       if (!timeout) {
+               pr_err("dmabuf wait for exclusive fence timed out.\n");
+               timeout = -ETIME;
+       }
+       err = timeout > 0 ? 0 : timeout;
+       dma_buf_unmap_attachment(import_attach, st, DMA_BIDIRECTIONAL);
+out_detach:
+       dma_buf_detach(dmabuf, import_attach);
+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 +392,7 @@ 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









