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

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

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

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