gpu: nvgpu: test free user vidmem atomically

An empty list of soon-to-be-freed userspace vidmem buffers is not enough
to safely assume that an allocation may succeed or not if tried again,
because removal from the list and actually marking the memory freed is
not atomic. Fix this by using an atomic counter for the number of
pending frees (so that it's still safe to first remove from the job list
and then perform the free), and making allocation attempts combined with
a test of pending frees atomic.

This still does not guarantee that there is memory available (as the
actual amount of pending memory in bytes plus the current free amount
isn't computed), but removes the race that produces false negatives in
case a single program expects repeated frees and allocs to succeed.

Bug 1809939

Change-Id: I6a92da2e21cbf3f886b727000c924d56f35ce55b
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: http://git-master/r/1217078
(cherry picked from commit 83c1f1e70dccd92fdd4481132cf5b6717760d432)
Reviewed-on: http://git-master/r/1220545
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/gpu/nvgpu/gk20a/mm_gk20a.h b/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
index 9002290e..8ce110a1 100644
--- a/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
+++ b/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
@@ -420,8 +420,10 @@ struct mm_gk20a {
 
                 struct list_head clear_list_head;
                 struct mutex clear_list_mutex;
+
+                struct work_struct clear_mem_worker;
+                atomic_t clears_pending;
         } vidmem;
-        struct work_struct vidmem_clear_mem_worker;
 };
 
 int gk20a_mm_init(struct mm_gk20a *mm);