linux-kernel - Re: [PATCH] locking/mutex: remove redundant argument from __mutex_lock

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Date:   Wed, 30 Aug 2023 19:54:13 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/mutex: remove redundant argument from
 __mutex_lock_common()

On 8/30/23 18:12, Michał Mirosław wrote:
> use_ww_ctx is equivalent to ww_ctx != NULL. The one case where
> use_ww_ctx was true but ww_ctx == NULL leads to the same
> __mutex_add_waiter() call via __ww_mutex_add_waiter().
I think ww_mutex_lock() can be called with a NULL ctx. Your patch will 
effectively change those ww_mutex_lock() to be equivalent to 
mutex_lock(). So it is a behavioral change.
> Since now __ww_mutex_add_waiter() is called only with ww_mutex != NULL,
> remove the branch there.
>
> Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> ---
>   kernel/locking/mutex.c    | 16 ++++++----------
>   kernel/locking/ww_mutex.h |  5 -----
>   2 files changed, 6 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index d973fe6041bf..2f0e318233f5 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -568,15 +568,12 @@ EXPORT_SYMBOL(ww_mutex_unlock);
>   static __always_inline int __sched
>   __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclass,
>   		    struct lockdep_map *nest_lock, unsigned long ip,
> -		    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
> +		    struct ww_acquire_ctx *ww_ctx)
>   {
>   	struct mutex_waiter waiter;
>   	struct ww_mutex *ww;
>   	int ret;
>   
> -	if (!use_ww_ctx)
> -		ww_ctx = NULL;
> -
That code is probably not needed given the current usage. Perhaps, you 
can change it to "WARN_ON_ONCE(ww_ctx && !use_ww_ctx);"
>   	might_sleep();
>   
>   	MUTEX_WARN_ON(lock->magic != lock);
> @@ -627,12 +624,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
>   
>   	debug_mutex_lock_common(lock, &waiter);
>   	waiter.task = current;
> -	if (use_ww_ctx)
> -		waiter.ww_ctx = ww_ctx;
> +	waiter.ww_ctx = ww_ctx;
This one is fine.
>   
>   	lock_contended(&lock->dep_map, ip);
>   
> -	if (!use_ww_ctx) {
> +	if (!ww_ctx) {
That change will break ww_mutex.

Cheers,
Longman