[PATCH RFC tip/core/rcu 06/14] rcu: Make synchronize_rcu_expedited() use sequence-counter scheme

Paul E. McKenney
2015-06-30T22:27:54Z

Although synchronize_rcu_expedited() uses a sequence-counter scheme, it
is based on a single increment per grace period, which means that tasks
piggybacking off of concurrent grace periods may be forced to wait longer
than necessary. This commit therefore applies the new sequence-count
functions developed for synchronize_sched_expedited() to speed things
up a bit and to consolidate the sequence-counter implementation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/tree_plugin.h | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 07bd9fb393b4..190213ffad19 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -536,7 +536,6 @@ void synchronize_rcu(void)
EXPORT_SYMBOL_GPL(synchronize_rcu);

static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
-static unsigned long sync_rcu_preempt_exp_count;
static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);

/*
@@ -704,12 +703,10 @@ void synchronize_rcu_expedited(void)
{
struct rcu_node *rnp;
struct rcu_state *rsp = rcu_state_p;
- unsigned long snap;
+ unsigned long s;
int trycount = 0;

- smp_mb(); /* Caller's modifications seen first by other CPUs. */
- snap = READ_ONCE(sync_rcu_preempt_exp_count) + 1;
- smp_mb(); /* Above access cannot bleed into critical section. */
+ s = rcu_exp_gp_seq_snap(rsp);

/*
* Acquire lock, falling back to synchronize_rcu() if too many
@@ -717,8 +714,7 @@ void synchronize_rcu_expedited(void)
* expedited grace period for us, just leave.
*/
while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
- if (ULONG_CMP_LT(snap,
- READ_ONCE(sync_rcu_preempt_exp_count)))
+ if (rcu_exp_gp_seq_done(rsp, s))
goto mb_ret; /* Others did our work for us. */
if (trycount++ < 10) {
udelay(trycount * num_online_cpus());
@@ -727,8 +723,9 @@ void synchronize_rcu_expedited(void)
return;
}
}
- if (ULONG_CMP_LT(snap, READ_ONCE(sync_rcu_preempt_exp_count)))
+ if (rcu_exp_gp_seq_done(rsp, s))
goto unlock_mb_ret; /* Others did our work for us. */
+ rcu_exp_gp_seq_start(rsp);

/* force all RCU readers onto ->blkd_tasks lists. */
synchronize_sched_expedited();
@@ -750,8 +747,7 @@ void synchronize_rcu_expedited(void)
sync_rcu_preempt_exp_done(rnp));

/* Clean up and exit. */
- smp_mb(); /* ensure expedited GP seen before counter increment. */
- WRITE_ONCE(sync_rcu_preempt_exp_count, sync_rcu_preempt_exp_count + 1);
+ rcu_exp_gp_seq_end(rsp);
unlock_mb_ret:
mutex_unlock(&sync_rcu_preempt_exp_mutex);
mb_ret:
--
1.8.1.5