Re: [PATCH v3 3/3] rcu: Use _full() API to debug synchronize_rcu()

Uladzislau Rezki
2025-02-26T20:48:56Z

Hello, Boqun.

> Hi Ulad,
>
> On Tue, Feb 25, 2025 at 12:00:20PM +0100, Uladzislau Rezki (Sony) wrote:
> > Switch for using of get_state_synchronize_rcu_full() and
> > poll_state_synchronize_rcu_full() pair for debug a normal
> > synchronize_rcu() call.
> >
> > Just using "not" full APIs to identify if a grace period
> > is passed or not might lead to a false kernel splat.
> >
>
> Could you provide detailed explanation on this? I.e. why is _full() is
> needed? I find the current commit message is a bit vague.
>
<snip>
rcu: Use _full() API to debug synchronize_rcu()

Switch for using of get_state_synchronize_rcu_full() and
poll_state_synchronize_rcu_full() pair to debug a normal
synchronize_rcu() call.

Just using "not" full APIs to identify if a grace period is
passed or not might lead to a false-positive kernel splat.

It can happen, because get_state_synchronize_rcu() compresses
both normal and expedited states into one single unsigned long
value, so a poll_state_synchronize_rcu() can miss GP-completion
when synchronize_rcu()/synchronize_rcu_expedited() concurrently
run.

To address this, switch to poll_state_synchronize_rcu_full() and
get_state_synchronize_rcu_full() APIs, which use separate variables
for expedited and normal states.
<snip>

Does it look better?

--
Uladzislau Rezki