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, 3 insertions, 0 deletions

diff --git a/drivers/gpu/nvgpu/common/linux/ioctl_ctrl.c b/drivers/gpu/nvgpu/common/linux/ioctl_ctrl.c
index 2c274f23..0d79b143 100644
--- a/drivers/gpu/nvgpu/common/linux/ioctl_ctrl.c
+++ b/drivers/gpu/nvgpu/common/linux/ioctl_ctrl.c
@@ -354,6 +354,8 @@ static int nvgpu_gpu_ioctl_inval_icache(
         nvgpu_mutex_acquire(&g->dbg_sessions_lock);
         err = g->ops.gr.inval_icache(g, ch);
         nvgpu_mutex_release(&g->dbg_sessions_lock);
+
+        gk20a_channel_put(ch);
         return err;
 }
 
@@ -393,6 +395,7 @@ static int nvgpu_gpu_ioctl_set_debug_mode(
                 err = -ENOSYS;
         nvgpu_mutex_release(&g->dbg_sessions_lock);
 
+        gk20a_channel_put(ch);
         return err;
 }