On 07/01/2015 10:59 AM, Chris Wilson wrote:
On Wed, Jul 01, 2015 at 10:48:59AM +0100, Tvrtko Ursulin wrote:
On 06/30/2015 05:55 PM, Chris Wilson wrote:
The userptr worker allows for a slight race condition where upon there
may two or more threads calling get_user_pages for the same object. When
we have the array of pages, then we serialise the update of the object.
However, the worker should only overwrite the obj->userptr.work pointer
if and only if it is the active one. Currently we clear it for a
secondary worker with the effect that we may rarely force a second
lookup.
Secondary worker can fire only if invalidate clears the current one,
no? (if (obj->userptr.work == NULL && ...))
It then "cancels" the worker so that the st_set_pages path is avoided.
I may have overegged the changelog, but what I did not like here was
that we would touch obj->userptr.work when we clearly had lost ownership
of that field.
Yes that part makes sense.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem_userptr.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 7a5242cd5ea5..cb367d9f7909 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -581,17 +581,17 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
}
mutex_lock(&dev->struct_mutex);
- if (obj->userptr.work != &work->work) {
- ret = 0;
- } else if (pinned == num_pages) {
- ret = st_set_pages(&obj->pages, pvec, num_pages);
- if (ret == 0) {
- list_add_tail(&obj->global_list, &to_i915(dev)->mm.unbound_list);
- pinned = 0;
+ if (obj->userptr.work == &work->work) {
+ if (pinned == num_pages) {
+ ret = st_set_pages(&obj->pages, pvec, num_pages);
+ if (ret == 0) {
+ list_add_tail(&obj->global_list, &to_i915(dev)->mm.unbound_list);
+ pinned = 0;
+ }
}
+ obj->userptr.work = ERR_PTR(ret);
}
- obj->userptr.work = ERR_PTR(ret);
obj->userptr.workers--;
drm_gem_object_unreference(&obj->base);
mutex_unlock(&dev->struct_mutex);
Previously the canceled worker would allow another worker to be
created in case it failed (obj->userptr.work != &work->work; ret =
0;) and now it still does since obj->userptr.work remains at NULL
from cancellation.
Both seem wrong, am I missing the change?
No, the obj->userptr.work must remain NULL until a new get_pages()
because we don't actually know if this worker's gup was before or after
the cancellation - mmap_sem vs struct_mutex ordering.
No one is not wrong, or no I was not missing the change?
I am thinking more and more that we should just mark it canceled forever
and not allow get_pages to succeed ever since.
Regards,
Tvrtko
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