Back in 2011, a lock (${TMPDIR}/kernel-scripts.lock) was introduced to protect do_make_scripts() from racing with itself when executed from different module recipes. The race was due to some of the kernel scripts always being rebuilt. 

Unfortunately this lock is not enough, since there is still a race between the scripts being rebuilt for one recipe and them being used by another. I.e., if do_make_script() is being executed for one kernel module recipe while another kernel module recipe is compiling, then there is a tiny chance that, e.g., modpost has been rewritten but not yet made executable by the first task at the same time as the second task tries to execute modpost, resulting in an error like this:

  /bin/sh: scripts/mod/modpost: Permission denied

My suggestion for a solution is to move do_make_scripts() from modules-base.bbclass to a recipe of its own (e.g., kernel-scripts) and then let modules.bbclass add a dependency on that recipe instead of adding the make_scripts task to the task list for the inheriting recipe. That way there should only ever be one recipe and task that creates the kernel scripts, and no race exists any more (which also means the lock file is no longer needed).