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 <[email protected]>
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
>
