On Mon, Mar 25, 2024 at 06:39:08PM +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@xxxxxxxxx>
>
> When using "guard(rcu)();" sparse will complain, because even
> though it now understands the cleanup attribute, it doesn't
> evaluate the calls from it at function exit, and thus doesn't
> count the context correctly.
>
> Given that there's a conditional in the resulting code:
>
> static inline void class_rcu_destructor(class_rcu_t *_T)
> {
> if (_T->lock) {
> rcu_read_unlock();
> }
> }
>
> it seems that even trying to teach sparse to evalulate the
> cleanup attribute function it'd still be difficult to really
> make it understand the full context here.
>
> Suppress the sparse warning by just releasing the context in
> the acquisition part of the function, after all we know it's
> safe with the guard, that's the whole point of it.
>
> Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Regards,
Boqun
> ---
> v2: add a comment after discussion with Boqun
>
> ---
> include/linux/rcupdate.h | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 0746b1b0b663..6a3c52b3c180 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1059,6 +1059,18 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
> extern int rcu_expedited;
> extern int rcu_normal;
>
> -DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
> +DEFINE_LOCK_GUARD_0(rcu,
> + do {
> + rcu_read_lock();
> + /*
> + * sparse doesn't call the cleanup function,
> + * so just release immediately and don't track
> + * the context. We don't need to anyway, since
> + * the whole point of the guard is to not need
> + * the explicit unlock.
> + */
> + __release(RCU);
> + } while(0),
> + rcu_read_unlock())
>
> #endif /* __LINUX_RCUPDATE_H */
> --
> 2.44.0
>
