Re: [PATCH] locking/ww_mutex: Initialize waiter.ww_ctx properly

[Peter Zijlstra]

On Thu, Aug 19, 2021 at 08:08:50PM +0200, Sebastian Andrzej Siewior wrote:
> The gathering of the debug code for the ww-mutex initialized moved the
> POISON initialiation into one spot and only set waiter.ww_ctx if the
> ww_ctx was non-NULL thus keeping the POISON value in ww-mutex case.
> 
> For ww-mutex without a context it is expected to set the context to
> NULL, the poison value was intended only for the regular mutex.
> 
> Move the waiter.ww_ctx where it was previously so it is initialized to
> NULL if no ww_ctx was passed.
> 
> Fixes: c0afb0ffc06e6 ("locking/ww_mutex: Gather mutex_waiter initialization")
> Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  kernel/locking/mutex.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 05b68931622d1..d1068b0b163af 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -614,8 +614,6 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
>  
>  	debug_mutex_lock_common(lock, &waiter);
>  	waiter.task = current;
> -	if (ww_ctx)
> -		waiter.ww_ctx = ww_ctx;
>  
>  	lock_contended(&lock->dep_map, ip);
>  
> @@ -630,6 +628,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
>  		ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
>  		if (ret)
>  			goto err_early_kill;
> +		waiter.ww_ctx = ww_ctx;

Yeah, I suppose this works too, but I wanted to keep the waiter
initialization together. Note how you've already called
__ww_mutex_add_waiter() on it before you actually set part of the waiter
state.