Re: [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic formisaligned rcu_head structures

[Josh Triplett]

On Wed, Feb 01, 2012 at 11:41:22AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 
> The push for energy efficiency will require that RCU tag rcu_head
> structures to indicate whether or not their invocation is time critical.
> This tagging is best carried out in the bottom bits of the ->next
> pointers in the rcu_head structures.  This tagging requires that the
> rcu_head structures be properly aligned, so this commit adds the required
> diagnostics.
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcutree.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index d0f8e64..4655f3e 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1707,6 +1707,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
>  	unsigned long flags;
>  	struct rcu_data *rdp;
>  
> +	WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
>  	debug_rcu_head_queue(head);

Upon further checking:

static inline void debug_rcu_head_queue(struct rcu_head *head)
{
        WARN_ON_ONCE((unsigned long)head & 0x3);

:)

- Josh Triplett
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