On Wed, 13 Oct 2021 10:04:52 +0800
王贇 [off-list 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