linux-kernel - Re: [PATCH] doc: Update the comparisons rule in rcu

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Message-Id: <20170321180950.GN3637@linux.vnet.ibm.com>
Date:   Tue, 21 Mar 2017 11:09:50 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Michalis Kokologiannakis <mixaskok@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] doc: Update the comparisons rule in rcu_dereference.txt

On Mon, Mar 20, 2017 at 10:38:35PM +0100, Michalis Kokologiannakis wrote:
> When an RCU-protected pointer is fetched but never dereferenced
> rcu_access_pointer() should be used in place of rcu_dereference().
> This commit explicitly records this very fact in Documentation/
> RCU/rcu_dereference.txt, in order to prevent the usage of
> rcu_dereference() in comparisons.
> 
> Signed-off-by: Michalis Kokologiannakis <mixaskok@...il.com>

Queued for review, thank you!

							Thanx, Paul

> ---
>  Documentation/RCU/rcu_dereference.txt | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
> index c0bf244..b2a613f 100644
> --- a/Documentation/RCU/rcu_dereference.txt
> +++ b/Documentation/RCU/rcu_dereference.txt
> @@ -138,6 +138,15 @@ o	Be very careful about comparing pointers obtained from
>  		This sort of comparison occurs frequently when scanning
>  		RCU-protected circular linked lists.
> 
> +		Note that if checks for being within an RCU read-side
> +		critical section are not required and the pointer is never
> +		dereferenced, rcu_access_pointer() should be used in place
> +		of rcu_dereference(). The rcu_access_pointer() primitive
> +		does not require an enclosing read-side critical section,
> +		and also omits the smp_read_barrier_depends() included in
> +		rcu_dereference(), which in turn should provide a small
> +		performance gain in some CPUs (e.g., the DEC Alpha).
> +
>  	o	The comparison is against a pointer that references memory
>  		that was initialized "a long time ago."  The reason
>  		this is safe is that even if misordering occurs, the
> -- 
> 2.1.4
>