When handling execbuf relocations, we play a delicate dance with
pagefault. We first try to access the user pages underneath our
struct_mutex. However, if those pages were inside a GEM object, we may
trigger a pagefault and deadlock as i915_gem_fault() tries to
recursively acquire struct_mutex. Instead, we choose to disable
pagefaulting around the copy_from_user whilst inside the struct_mutex
and handle the EFAULT by falling back to a copy outside the
struct_mutex.
We however presumed that disabling pagefaults would be expensive. It is
just an operation on the local current task. Cheap enough that we can
restrict the disable/enable to the critical section around the copy, and
so avoid having to handle the atomic sections within the relocation
handling itself.
v2: Just illustrate the broken error handling rather than argue why it
is safer to ignore it, for now.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem_execbuffer.c | 71 +++++++++++++++---------------
1 file changed, 35 insertions(+), 36 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 204536a3087e..e52affdcc125 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -551,20 +551,6 @@ relocate_entry(struct drm_i915_gem_object *obj,
return 0;
}
-static bool object_is_idle(struct drm_i915_gem_object *obj)
-{
- unsigned long active = i915_gem_object_get_active(obj);
- int idx;
-
- for_each_active(active, idx) {
- if (!i915_gem_active_is_idle(&obj->last_read[idx],
- &obj->base.dev->struct_mutex))
- return false;
- }
-
- return true;
-}
-
static int
i915_gem_execbuffer_relocate_entry(struct drm_i915_gem_object *obj,
struct eb_vmas *eb,
@@ -648,10 +634,6 @@ i915_gem_execbuffer_relocate_entry(struct drm_i915_gem_object *obj,
return -EINVAL;
}
- /* We can't wait for rendering with pagefaults disabled */
- if (pagefault_disabled() && !object_is_idle(obj))
- return -EFAULT;
-
ret = relocate_entry(obj, reloc, cache, target_offset);
if (ret)
return ret;
@@ -678,12 +660,23 @@ i915_gem_execbuffer_relocate_vma(struct i915_vma *vma,
remain = entry->relocation_count;
while (remain) {
struct drm_i915_gem_relocation_entry *r = stack_reloc;
- int count = remain;
- if (count > ARRAY_SIZE(stack_reloc))
- count = ARRAY_SIZE(stack_reloc);
+ unsigned long unwritten;
+ unsigned int count;
+
+ count = min_t(unsigned int, remain, ARRAY_SIZE(stack_reloc));
remain -= count;
- if (__copy_from_user_inatomic(r, user_relocs, count*sizeof(r[0]))) {
+ /* This is the fast path and we cannot handle a pagefault
+ * whilst holding the struct mutex lest the user pass in the
+ * relocations contained within a mmaped bo. For in such a case
+ * we, the page fault handler would call i915_gem_fault() and
+ * we would try to acquire the struct mutex again. Obviously
+ * this is bad and so lockdep complains vehemently.
+ */
+ pagefault_disable();
+ unwritten = __copy_from_user_inatomic(r, user_relocs, count*sizeof(r[0]));
+ pagefault_enable();
+ if (unlikely(unwritten)) {
ret = -EFAULT;
goto out;
}
@@ -695,11 +688,26 @@ i915_gem_execbuffer_relocate_vma(struct i915_vma *vma,
if (ret)
goto out;
- if (r->presumed_offset != offset &&
- __put_user(r->presumed_offset,
- &user_relocs->presumed_offset)) {
- ret = -EFAULT;
- goto out;
+ if (r->presumed_offset != offset) {
+ pagefault_disable();
+ unwritten = __put_user(r->presumed_offset,
+ &user_relocs->presumed_offset);
+ pagefault_enable();
+ if (unlikely(unwritten)) {
+ /* Note that reporting an error now
+ * leaves everything in an inconsistent
+ * state as we have *already* changed
+ * the relocation value inside the
+ * object. As we have not changed the
+ * reloc.presumed_offset or will not
+ * change the execobject.offset, on the
+ * call we may not rewrite the value
+ * inside the object, leaving it
+ * dangling and causing a GPU hang.
+ */
+ ret = -EFAULT;
+ goto out;
+ }
}
user_relocs++;
@@ -739,20 +747,11 @@ i915_gem_execbuffer_relocate(struct eb_vmas *eb)
struct i915_vma *vma;
int ret = 0;
- /* This is the fast path and we cannot handle a pagefault whilst
- * holding the struct mutex lest the user pass in the relocations
- * contained within a mmaped bo. For in such a case we, the page
- * fault handler would call i915_gem_fault() and we would try to
- * acquire the struct mutex again. Obviously this is bad and so
- * lockdep complains vehemently.
- */
- pagefault_disable();
list_for_each_entry(vma, &eb->vmas, exec_list) {
ret = i915_gem_execbuffer_relocate_vma(vma, eb);
if (ret)
break;
}
- pagefault_enable();
return ret;
}
