Re: [PATCH v2] rcu: mollify sparse with RCU guard

[Boqun Feng]

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
>