gpu: nvgpu: hold ch ref when getting ch from fd

Fix a race condition in gk20a_get_channel_from_file() that returns a
channel pointer from an fd: take a reference to the channel before
putting the file ref back. Now the caller is responsible of releasing
the channel reference eventually.

Also document why dbg_session_channel_data has to hold a ref to the
channel file instead of just the channel: that might deadlock if the fds
were closed in "wrong" order.

Change-Id: I8e91b809f5f7b1cb0c1487bd955ad6d643727a53
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1549290
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/drivers/gpu/nvgpu/common/linux/ioctl_as.c b/drivers/gpu/nvgpu/common/linux/ioctl_as.c
index 6fd0a3d2..d4242955 100644
--- a/drivers/gpu/nvgpu/common/linux/ioctl_as.c
+++ b/drivers/gpu/nvgpu/common/linux/ioctl_as.c
@@ -42,14 +42,19 @@ static int gk20a_as_ioctl_bind_channel(
         gk20a_dbg_fn("");
 
         ch = gk20a_get_channel_from_file(args->channel_fd);
-        if (!ch || gk20a_channel_as_bound(ch))
+        if (!ch)
                 return -EINVAL;
 
+        if (gk20a_channel_as_bound(ch)) {
+                err = -EINVAL;
+                goto out;
+        }
+
         /* this will set channel_gk20a->vm */
         err = ch->g->ops.mm.vm_bind_channel(as_share, ch);
-        if (err)
-                return err;
 
+out:
+        gk20a_channel_put(ch);
         return err;
 }