linux-kernel - Re: [RFC PATCH 09/21] list: Remove unnecessary WRITE_ONCE() from hlist_bl_add

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Date:   Mon, 30 Mar 2020 16:30:20 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Maddie Stone <maddiestone@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 09/21] list: Remove unnecessary WRITE_ONCE() from
 hlist_bl_add_before()

On Tue, Mar 24, 2020 at 03:36:31PM +0000, Will Deacon wrote:
> There is no need to use WRITE_ONCE() when inserting into a non-RCU
> protected list.
> 
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will@...nel.org>

OK, I will bite...  Why "unsigned long" instead of "uintptr_t"?

Not that it matters in the context of the Linux kernel, so:

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

							Thanx, Paul

> ---
>  include/linux/list_bl.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
> index 1804fdb84dda..c93e7e3aa8fc 100644
> --- a/include/linux/list_bl.h
> +++ b/include/linux/list_bl.h
> @@ -91,15 +91,15 @@ static inline void hlist_bl_add_before(struct hlist_bl_node *n,
>  				       struct hlist_bl_node *next)
>  {
>  	struct hlist_bl_node **pprev = next->pprev;
> +	unsigned long val;
>  
>  	n->pprev = pprev;
>  	n->next = next;
>  	next->pprev = &n->next;
>  
>  	/* pprev may be `first`, so be careful not to lose the lock bit */
> -	WRITE_ONCE(*pprev,
> -		   (struct hlist_bl_node *)
> -			((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK)));
> +	val = (unsigned long)n | ((unsigned long)*pprev & LIST_BL_LOCKMASK);
> +	*pprev = (struct hlist_bl_node *)val;
>  }
>  
>  static inline void hlist_bl_add_behind(struct hlist_bl_node *n,
> -- 
> 2.20.1
>