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Date: Tue, 5 Aug 2008 12:48:41 -0400 (EDT)
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc: linux-kernel@...r.kernel.org, mingo@...e.hu,
akpm@...ux-foundation.org, oleg@...sign.ru,
manfred@...orfullife.com, dipankar@...ibm.com, dvhltc@...ibm.com,
niv@...ibm.com
Subject: Re: [PATCH tip/core/rcu] classic RCU locking and memory-barrier
cleanups
On Tue, 5 Aug 2008, Paul E. McKenney wrote:
> ---
>
> rcuclassic.c | 51 +++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 41 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
> index d3553ee..e6f42ef 100644
> --- a/kernel/rcuclassic.c
> +++ b/kernel/rcuclassic.c
> @@ -86,6 +86,7 @@ static void force_quiescent_state(struct rcu_data *rdp,
> int cpu;
> cpumask_t cpumask;
> set_need_resched();
> + spin_lock(&rcp->lock);
> if (unlikely(!rcp->signaled)) {
> rcp->signaled = 1;
> /*
> @@ -111,6 +112,7 @@ static void force_quiescent_state(struct rcu_data *rdp,
> for_each_cpu_mask(cpu, cpumask)
> smp_send_reschedule(cpu);
> }
> + spin_unlock(&rcp->lock);
> }
> #else
> static inline void force_quiescent_state(struct rcu_data *rdp,
> @@ -124,7 +126,9 @@ static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
> struct rcu_data *rdp)
> {
> long batch;
> - smp_mb(); /* reads the most recently updated value of rcu->cur. */
> +
> + head->next = NULL;
> + smp_mb(); /* Read of rcu->cur must happen after any change by caller. */
>
> /*
> * Determine the batch number of this callback.
> @@ -174,7 +178,6 @@ void call_rcu(struct rcu_head *head,
> unsigned long flags;
>
> head->func = func;
> - head->next = NULL;
> local_irq_save(flags);
> __call_rcu(head, &rcu_ctrlblk, &__get_cpu_var(rcu_data));
> local_irq_restore(flags);
> @@ -203,7 +206,6 @@ void call_rcu_bh(struct rcu_head *head,
> unsigned long flags;
>
> head->func = func;
> - head->next = NULL;
> local_irq_save(flags);
> __call_rcu(head, &rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
> local_irq_restore(flags);
> @@ -389,17 +391,17 @@ static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
> static void __rcu_offline_cpu(struct rcu_data *this_rdp,
> struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> {
> - /* if the cpu going offline owns the grace period
> + /*
> + * if the cpu going offline owns the grace period
> * we can block indefinitely waiting for it, so flush
> * it here
> */
> spin_lock_bh(&rcp->lock);
> if (rcp->cur != rcp->completed)
> cpu_quiet(rdp->cpu, rcp);
> - spin_unlock_bh(&rcp->lock);
> - /* spin_lock implies smp_mb() */
The spin_unlock is being removed here. Was that the smp_mb that was
implied or is it the above spin_lock_bh?
> rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail, rcp->cur + 1);
> rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail[2], rcp->cur + 1);
> + spin_unlock_bh(&rcp->lock);
>
> local_irq_disable();
> this_rdp->qlen += rdp->qlen;
> @@ -433,16 +435,19 @@ static void rcu_offline_cpu(int cpu)
> static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
> struct rcu_data *rdp)
> {
> + long completed_snap;
> +
> if (rdp->nxtlist) {
> local_irq_disable();
> + completed_snap = ACCESS_ONCE(rcp->completed);
>
> /*
> * move the other grace-period-completed entries to
> * [rdp->nxtlist, *rdp->nxttail[0]) temporarily
> */
> - if (!rcu_batch_before(rcp->completed, rdp->batch))
> + if (!rcu_batch_before(completed_snap, rdp->batch))
> rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2];
> - else if (!rcu_batch_before(rcp->completed, rdp->batch - 1))
> + else if (!rcu_batch_before(completed_snap, rdp->batch - 1))
> rdp->nxttail[0] = rdp->nxttail[1];
>
> /*
> @@ -483,20 +488,38 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
>
> static void rcu_process_callbacks(struct softirq_action *unused)
> {
> + /*
> + * Memory references from any prior RCU read-side critical sections
> + * executed by the interrupted code must be see before any RCU
> + * grace-period manupulations below.
> + */
> +
> + smp_mb(); /* See above block comment. */
> +
> __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
> __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
> +
> + /*
> + * Memory references from any later RCU read-side critical sections
> + * executed by the interrupted code must be see after any RCU
> + * grace-period manupulations above.
> + */
> +
> + smp_mb(); /* See above block comment. */
> }
>
> static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> {
> if (rdp->nxtlist) {
> + long completed_snap = ACCESS_ONCE(rcp->completed);
> +
> /*
> * This cpu has pending rcu entries and the grace period
> * for them has completed.
> */
> - if (!rcu_batch_before(rcp->completed, rdp->batch))
> + if (!rcu_batch_before(completed_snap, rdp->batch))
> return 1;
> - if (!rcu_batch_before(rcp->completed, rdp->batch - 1) &&
> + if (!rcu_batch_before(completed_snap, rdp->batch - 1) &&
> rdp->nxttail[0] != rdp->nxttail[1])
> return 1;
> if (rdp->nxttail[0] != &rdp->nxtlist)
> @@ -547,6 +570,12 @@ int rcu_needs_cpu(int cpu)
> return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu);
> }
>
> +/*
> + * Top-level function driving RCU grace-period detection, normally
> + * invoked from the scheduler-clock interrupt. This function simply
> + * increments counters that are read only from softirq by this same
> + * CPU, so there are no memory barriers required.
> + */
> void rcu_check_callbacks(int cpu, int user)
> {
> if (user ||
> @@ -590,6 +619,7 @@ void rcu_check_callbacks(int cpu, int user)
> static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
> struct rcu_data *rdp)
> {
> + spin_lock(&rcp->lock);
> memset(rdp, 0, sizeof(*rdp));
> rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2] = &rdp->nxtlist;
> rdp->donetail = &rdp->donelist;
> @@ -597,6 +627,7 @@ static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
> rdp->qs_pending = 0;
> rdp->cpu = cpu;
> rdp->blimit = blimit;
> + spin_unlock(&rcp->lock);
> }
>
> static void __cpuinit rcu_online_cpu(int cpu)
>
--
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