[PATCH 1/3] dma_resv: prime lockdep annotations

Daniel Vetter daniel.vetter at ffwll.ch
Wed Aug 21 21:50:28 UTC 2019

Full audit of everyone:

- i915, radeon, amdgpu should be clean per their maintainers.

- vram helpers should be fine, they don't do command submission, so
  really no business holding struct_mutex while doing copy_*_user. But
  I haven't checked them all.

- panfrost seems to dma_resv_lock only in panfrost_job_push, which
  looks clean.

- v3d holds dma_resv locks in the tail of its v3d_submit_cl_ioctl(),
  copying from/to userspace happens all in v3d_lookup_bos which is
  outside of the critical section.

- vmwgfx has a bunch of ioctls that do their own copy_*_user:
  - vmw_execbuf_process: First this does some copies in
    vmw_execbuf_cmdbuf() and also in the vmw_execbuf_process() itself.
    Then comes the usual ttm reserve/validate sequence, then actual
    submission/fencing, then unreserving, and finally some more
    copy_to_user in vmw_execbuf_copy_fence_user. Glossing over tons of
    details, but looks all safe.
  - vmw_fence_event_ioctl: No ttm_reserve/dma_resv_lock anywhere to be
    seen, seems to only create a fence and copy it out.
  - a pile of smaller ioctl in vmwgfx_ioctl.c, no reservations to be
    found there.
  Summary: vmwgfx seems to be fine too.

- virtio: There's virtio_gpu_execbuffer_ioctl, which does all the
  copying from userspace before even looking up objects through their
  handles, so safe. Plus the getparam/getcaps ioctl, also both safe.

- qxl only has qxl_execbuffer_ioctl, which calls into
  qxl_process_single_command. There's a lovely comment before the
  __copy_from_user_inatomic that the slowpath should be copied from
  i915, but I guess that never happened. Try not to be unlucky and get
  your CS data evicted between when it's written and the kernel tries
  to read it. The only other copy_from_user is for relocs, but those
  are done before qxl_release_reserve_list(), which seems to be the
  only thing reserving buffers (in the ttm/dma_resv sense) in that
  code. So looks safe.

- A debugfs file in nouveau_debugfs_pstate_set() and the usif ioctl in
  usif_ioctl() look safe. nouveau_gem_ioctl_pushbuf() otoh breaks this
  everywhere and needs to be fixed up.

v2: Thomas pointed at that vmwgfx calls dma_resv_init while it holds a
dma_resv lock of a different object already. Christian mentioned that
ttm core does this too for ghost objects. intel-gfx-ci highlighted
that i915 has similar issues.

Unfortunately we can't do this in the usual module init functions,
because kernel threads don't have an ->mm - we have to wait around for
some user thread to do this.

Solution is to spawn a worker (but only once). It's horrible, but it

Cc: Alex Deucher <alexander.deucher at amd.com>
Cc: Christian K├Ânig <christian.koenig at amd.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: Rob Herring <robh at kernel.org>
Cc: Tomeu Vizoso <tomeu.vizoso at collabora.com>
Cc: Eric Anholt <eric at anholt.net>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Gerd Hoffmann <kraxel at redhat.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: "VMware Graphics" <linux-graphics-maintainer at vmware.com>
Cc: Thomas Hellstrom <thellstrom at vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
 drivers/dma-buf/dma-resv.c | 42 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 42a8f3f11681..29988b1564c1 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -34,6 +34,7 @@
 #include <linux/dma-resv.h>
 #include <linux/export.h>
+#include <linux/sched/mm.h>
  * DOC: Reservation Object Overview
@@ -95,6 +96,28 @@ static void dma_resv_list_free(struct dma_resv_list *list)
 	kfree_rcu(list, rcu);
+struct lockdep_work {
+	struct work_struct work;
+	struct dma_resv obj;
+	struct mm_struct *mm;
+} lockdep_work;
+void lockdep_work_fn(struct work_struct *work)
+	dma_resv_init(&lockdep_work.obj);
+	down_read(&lockdep_work.mm->mmap_sem);
+	ww_mutex_lock(&lockdep_work.obj.lock, NULL);
+	fs_reclaim_acquire(GFP_KERNEL);
+	fs_reclaim_release(GFP_KERNEL);
+	ww_mutex_unlock(&lockdep_work.obj.lock);
+	up_read(&lockdep_work.mm->mmap_sem);
+	mmput(lockdep_work.mm);
  * dma_resv_init - initialize a reservation object
  * @obj: the reservation object
@@ -107,6 +130,25 @@ void dma_resv_init(struct dma_resv *obj)
 	RCU_INIT_POINTER(obj->fence, NULL);
 	RCU_INIT_POINTER(obj->fence_excl, NULL);
+	if (current->mm) {
+		static atomic_t lockdep_primed;
+		/*
+		 * This gets called from all kinds of places, launch a worker.
+		 * Usual init sections don't work for kernel threads lack an
+		 * ->mm.
+		 */
+		if (atomic_cmpxchg(&lockdep_primed, 0, 1) == 0) {
+			INIT_WORK(&lockdep_work.work, lockdep_work_fn);
+			lockdep_work.mm = current->mm;
+			mmget(lockdep_work.mm);
+			schedule_work(&lockdep_work.work);
+		}
+	}

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