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SubjectRe: [PATCH tip/core/urgent] rcu: Stop rcu_do_batch() from multiplexing the "count" variable
On Fri, Jun 22, 2012 at 08:37:38AM -0700, Paul E. McKenney wrote:
> Commit b1420f1c (Make rcu_barrier() less disruptive) rearranged the
> code in rcu_do_batch(), moving the ->qlen manipulation to follow
> the requeueing of the callbacks. Unfortunately, this rearrangement
> clobbered the value of the "count" local variable before the value
> of rdp->qlen was adjusted, resulting in the value of rdp->qlen being
> inaccurate. This commit therefore introduces an index variable "i",
> avoiding the inadvertent multiplexing.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 2f34d02..2001d38 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1593,7 +1593,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> {
> unsigned long flags;
> struct rcu_head *next, *list, **tail;
> - int bl, count, count_lazy;
> + int bl, count, count_lazy, i;
>
> /* If no callbacks are ready, just return.*/
> if (!cpu_has_callbacks_ready_to_invoke(rdp)) {
> @@ -1616,9 +1616,9 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
> *rdp->nxttail[RCU_DONE_TAIL] = NULL;
> tail = rdp->nxttail[RCU_DONE_TAIL];
> - for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)
> - if (rdp->nxttail[count] == rdp->nxttail[RCU_DONE_TAIL])
> - rdp->nxttail[count] = &rdp->nxtlist;
> + for (i = RCU_NEXT_SIZE - 1; i >= 0; i--)
> + if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
> + rdp->nxttail[i] = &rdp->nxtlist;
> local_irq_restore(flags);
>
> /* Invoke callbacks. */
> @@ -1646,9 +1646,9 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> if (list != NULL) {
> *tail = rdp->nxtlist;
> rdp->nxtlist = list;
> - for (count = 0; count < RCU_NEXT_SIZE; count++)
> - if (&rdp->nxtlist == rdp->nxttail[count])
> - rdp->nxttail[count] = tail;
> + for (i = 0; i < RCU_NEXT_SIZE; i++)
> + if (&rdp->nxtlist == rdp->nxttail[i])
> + rdp->nxttail[i] = tail;
> else
> break;
> }
>


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