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, 9 insertions, 1 deletions

diff --git a/drivers/gpu/nvgpu/common/linux/ioctl_channel.c b/drivers/gpu/nvgpu/common/linux/ioctl_channel.c
index ab02dc42..5e09c677 100644
--- a/drivers/gpu/nvgpu/common/linux/ioctl_channel.c
+++ b/drivers/gpu/nvgpu/common/linux/ioctl_channel.c
@@ -260,8 +260,15 @@ static int gk20a_init_error_notifier(struct channel_gk20a *ch,
         return 0;
 }
 
+/*
+ * This returns the channel with a reference. The caller must
+ * gk20a_channel_put() the ref back after use.
+ *
+ * NULL is returned if the channel was not found.
+ */
 struct channel_gk20a *gk20a_get_channel_from_file(int fd)
 {
+        struct channel_gk20a *ch;
         struct channel_priv *priv;
         struct file *f = fget(fd);
 
@@ -274,8 +281,9 @@ struct channel_gk20a *gk20a_get_channel_from_file(int fd)
         }
 
         priv = (struct channel_priv *)f->private_data;
+        ch = gk20a_channel_get(priv->c);
         fput(f);
-        return priv->c;
+        return ch;
 }
 
 int gk20a_channel_release(struct inode *inode, struct file *filp)