linux-kernel - Re: [patch V5 41/72] locking/ww_mutex: Gather mutex

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Date:   Thu, 19 Aug 2021 19:51:10 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Mike Galbraith <efault@....de>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [patch V5 41/72] locking/ww_mutex: Gather mutex_waiter
 initialization

On 2021-08-15 23:28:39 [+0200], Thomas Gleixner wrote:
> --- a/kernel/locking/mutex-debug.c
> +++ b/kernel/locking/mutex-debug.c
> @@ -30,6 +30,7 @@ void debug_mutex_lock_common(struct mute
>  	memset(waiter, MUTEX_DEBUG_INIT, sizeof(*waiter));
>  	waiter->magic = waiter;
>  	INIT_LIST_HEAD(&waiter->list);
> +	waiter->ww_ctx = MUTEX_POISON_WW_CTX;
>  }
>  
>  void debug_mutex_wake_waiter(struct mutex *lock, struct mutex_waiter *waiter)
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -980,17 +980,15 @@ static __always_inline int __sched
>  	}
>  
>  	debug_mutex_lock_common(lock, &waiter);
> +	waiter.task = current;
> +	if (ww_ctx)
> +		waiter.ww_ctx = ww_ctx;
>  
>  	lock_contended(&lock->dep_map, ip);
>  
>  	if (!use_ww_ctx) {
>  		/* add waiting tasks to the end of the waitqueue (FIFO): */
>  		__mutex_add_waiter(lock, &waiter, &lock->wait_list);
> -
> -
> -#ifdef CONFIG_DEBUG_MUTEXES
> -		waiter.ww_ctx = MUTEX_POISON_WW_CTX;
> -#endif

The crash, Guenter Roeck reported, is because now waiter.ww_ctx is
always initialized to poisen.
Previously in the ww-mutex case without a context it was set to NULL
in the next hunk.
Keeping the ww_ctx assigment in the next hunk seems to work.

>  	} else {
>  		/*
>  		 * Add in stamp order, waking up waiters that must kill
> @@ -999,12 +997,8 @@ static __always_inline int __sched
>  		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
>  		if (ret)
>  			goto err_early_kill;
> -
> -		waiter.ww_ctx = ww_ctx;
>  	}
>  
> -	waiter.task = current;
> -
>  	set_current_state(state);
>  	for (;;) {
>  		/*

Sebastian