linux-kernel - Re: [PATCH] rcu: mollify sparse with RCU guard

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Date: Mon, 25 Mar 2024 09:35:04 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Johannes Berg <johannes@...solutions.net>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH] rcu: mollify sparse with RCU guard

On Mon, Mar 25, 2024 at 11:16:27AM +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> 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@...el.com>
> ---
>  include/linux/rcupdate.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 17d7ed5f3ae6..41081ee9c9a7 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1090,6 +1090,6 @@ 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(); __release(RCU); } while(0), rcu_read_unlock())
>  

Hmm.. not a big fan of this. __release(RCU) following a rcu_read_lock()
is really confusing. Maybe we can introduce a _rcu_read_lock():

	void _rcu_read_lock(bool guard) {
		__rcu_read_lock();
		// Skip sparse annotation in "guard(rcu)()" to work
		// around sparse's lack of support of cleanup.
		if (!guard)
			__acquire(RCU);
		rcu_lock_acquire(...);
		...
	}

and normal rcu_read_lock() is just a _rcu_read_lock(false), RCU guard is
a _rcu_read_lock(true)?

But before that how does it looks if we don't fix this entirely? ;-)

Regards,
Boqun

>  #endif /* __LINUX_RCUPDATE_H */
> -- 
> 2.44.0
>