# Re: \[PATCH 1/2\] ftrace: disable preemption on the testing of recursion

- From: Steven Rostedt \<rostedt@goodmis.org\>
- Date: 2021-10-13 02:30:46 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2021/10/1539739
- Thread: https://ratatoskr.run/linuxppc-dev/2021/10/1538560/t.md

```text
On Wed, 13 Oct 2021 10:04:52 +0800
王贇 [ref] wrote:

> I see, while the user can still check smp_processor_id() after trylock
> return bit 0...

But preemption would have already been disabled. That's because a bit 0
means that a recursion check has already been made by a previous
caller and this one is nested, thus preemption is already disabled.
If bit is 0, then preemption had better be disabled as well.

-- Steve
```