linux-kernel - Re: [PATCH] locking/qspinlock: Always evaluate lockevent* non-event parameter once

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Date: Mon, 18 Mar 2024 21:41:32 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Waiman Long <longman@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] locking/qspinlock: Always evaluate lockevent* non-event
 parameter once

On Mon, Mar 18, 2024 at 08:50:04PM -0400, Waiman Long wrote:
> The inc parameter of lockevent_add() and the cond parameter of
> lockevent_cond_inc() are only evaluated when CONFIG_LOCK_EVENT_COUNTS
> is on. That can cause problem if those parameters are expressions
> with side effect like a "++". Fix this by evaluating those non-event
> parameters once even if CONFIG_LOCK_EVENT_COUNTS is off. This will also
> eliminate the need of the __maybe_unused attribute to the wait_early
> local variable in pv_wait_node().
> 
> Suggested-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Waiman Long <longman@...hat.com>

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> ---
>  kernel/locking/lock_events.h        | 4 ++--
>  kernel/locking/qspinlock_paravirt.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h
> index a6016b91803d..d2345e9c0190 100644
> --- a/kernel/locking/lock_events.h
> +++ b/kernel/locking/lock_events.h
> @@ -53,8 +53,8 @@ static inline void __lockevent_add(enum lock_events event, int inc)
>  #else  /* CONFIG_LOCK_EVENT_COUNTS */
>  
>  #define lockevent_inc(ev)
> -#define lockevent_add(ev, c)
> -#define lockevent_cond_inc(ev, c)
> +#define lockevent_add(ev, c)		do { (void)(c); } while (0)
> +#define lockevent_cond_inc(ev, c)	do { (void)(c); } while (0)
>  
>  #endif /* CONFIG_LOCK_EVENT_COUNTS */
>  
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index ae2b12f68b90..169950fe1aad 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -294,7 +294,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>  {
>  	struct pv_node *pn = (struct pv_node *)node;
>  	struct pv_node *pp = (struct pv_node *)prev;
> -	bool __maybe_unused wait_early;
> +	bool wait_early;
>  	int loop;
>  
>  	for (;;) {
> -- 
> 2.39.3
>