linux-kernel - Re: [PATCH v2] locking/mutex: Set and clear owner using WRITE

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Date:	Fri, 20 May 2016 19:01:43 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Terry Rudd <terry.rudd@....com>,
	Scott J Norton <scott.norton@....com>,
	Jason Low <jason.low2@...com>
Subject: Re: [PATCH v2] locking/mutex: Set and clear owner using WRITE_ONCE()

On 05/20/2016 06:19 PM, Jason Low wrote:
> The mutex owner can get read and written to locklessly.
> Use WRITE_ONCE when setting and clearing the owner field
> in order to avoid optimizations such as store tearing. This
> avoids situations where the owner field gets written to with
> multiple stores and another thread could concurrently read
> and use a partially written owner value.
>
> Signed-off-by: Jason Low<jason.low2@....com>
> Acked-by: Davidlohr Bueso<dave@...olabs.net>
> ---
>   kernel/locking/mutex-debug.h |  4 ++--
>   kernel/locking/mutex.h       | 10 ++++++++--
>   2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/mutex-debug.h b/kernel/locking/mutex-debug.h
> index 0799fd3..372e653 100644
> --- a/kernel/locking/mutex-debug.h
> +++ b/kernel/locking/mutex-debug.h
> @@ -29,12 +29,12 @@ extern void debug_mutex_init(struct mutex *lock, const char *name,
>
>   static inline void mutex_set_owner(struct mutex *lock)
>   {
> -	lock->owner = current;
> +	WRITE_ONCE(lock->owner, current);
>   }
>
>   static inline void mutex_clear_owner(struct mutex *lock)
>   {
> -	lock->owner = NULL;
> +	WRITE_ONCE(lock->owner, NULL);
>   }
>
>   #define spin_lock_mutex(lock, flags)			\
> diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
> index 5cda397..12f9619 100644
> --- a/kernel/locking/mutex.h
> +++ b/kernel/locking/mutex.h
> @@ -17,14 +17,20 @@
>   		__list_del((waiter)->list.prev, (waiter)->list.next)
>
>   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> +/*
> + * The mutex owner can get read and written to locklessly.
> + * We should use WRITE_ONCE when writing the owner value to
> + * avoid store tearing, otherwise, a thread could potentially
> + * read a partially written and incomplete owner value.
> + */
>   static inline void mutex_set_owner(struct mutex *lock)
>   {
> -	lock->owner = current;
> +	WRITE_ONCE(lock->owner, current);
>   }
>
>   static inline void mutex_clear_owner(struct mutex *lock)
>   {
> -	lock->owner = NULL;
> +	WRITE_ONCE(lock->owner, NULL);
>   }
>   #else
>   static inline void mutex_set_owner(struct mutex *lock)
Acked-by: Waiman Long <Waiman.Long@....com>