media: v4l2-ctrls: improve media_request_(un)lock_for_update

The request reference count was decreased again once a reference to the
request object was taken. Postpone this until we finished using the object.

In theory I think it is possible that the request_fd can be closed by
the application from another thread. In that case when request_put is
called the whole request would be freed.

It's highly unlikely, but let's just be safe and fix this potential
race condition.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
index cc266a4a6e88b83b22f3eabd818efe72e6aaa040..95d065d543081f522d92a572f3b54c0cd9566969 100644 (file)
--- a/drivers/media/v4l2-core/v4l2-ctrls.c
+++ b/drivers/media/v4l2-core/v4l2-ctrls.c
@@ -3657,10 +3657,9 @@ static int try_set_ext_ctrls(struct v4l2_fh *fh,
                }
 
                obj = v4l2_ctrls_find_req_obj(hdl, req, set);
-               /* Reference to the request held through obj */
-               media_request_put(req);
                if (IS_ERR(obj)) {
                        media_request_unlock_for_update(req);
+                       media_request_put(req);
                        return PTR_ERR(obj);
                }
                hdl = container_of(obj, struct v4l2_ctrl_handler,
@@ -3670,8 +3669,9 @@ static int try_set_ext_ctrls(struct v4l2_fh *fh,
        ret = try_set_ext_ctrls_common(fh, hdl, cs, set);
 
        if (obj) {
-               media_request_unlock_for_update(obj->req);
+               media_request_unlock_for_update(req);
                media_request_object_put(obj);
+               media_request_put(req);
        }
 
        return ret;