On Thu, 14 Oct 2021 17:14:07 +0200
Petr Mladek [off-list ref] wrote:

  /**
   * ftrace_test_recursion_trylock - tests for recursion in same context
   *
   * Use this for ftrace callbacks. This will detect if the function
   * tracing recursed in the same context (normal vs interrupt),
   *
   * Returns: -1 if a recursion happened.
-  *           >= 0 if no recursion
+  *           >= 0 if no recursion (success)
+  *
+  * Disables the preemption on success. It is just for a convenience.
+  * Current users needed to disable the preemtion for some reasons.
   */

I started replying to this explaining the difference between bit not
zero and a bit zero, and I think I found a design flaw that can allow
unwanted recursion.

It's late and I'm about to go to bed, but I may have a new patch to fix
this before this gets added, as the fix will conflict with this patch,
and the fix will likely need to go to stable.

Stay tuned.

-- Steve