[PATCH] rcu: eliminate rcu_data.last_qsctr

Oleg Nesterov
2004-11-28T16:47:05Z

last_qsctr is used in rcu_check_quiescent_state() exclusively.
We can reset qsctr at the start of the grace period, and then
just test qsctr against 0.

On top of the 'rcu: eliminate rcu_ctrlblk.lock', see
http://marc.theaimsgroup.com/?l=linux-kernel&m=110156786721526

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.10-rc2/include/linux/rcupdate.h~ 2004-11-27 21:32:49.000000000 +0300
+++ 2.6.10-rc2/include/linux/rcupdate.h 2004-11-28 23:15:20.510856936 +0300
@@ -88,8 +88,6 @@ struct rcu_data {
/* 1) quiescent state handling : */
long quiescbatch; /* Batch # for grace period */
long qsctr; /* User-mode/idle loop etc. */
- long last_qsctr; /* value of qsctr at beginning */
- /* of rcu grace period */
int qs_pending; /* core waits for quiesc state */

/* 2) batch handling */
--- 2.6.10-rc2/kernel/rcupdate.c~ 2004-11-28 23:13:03.324712400 +0300
+++ 2.6.10-rc2/kernel/rcupdate.c 2004-11-28 23:15:20.511856784 +0300
@@ -215,9 +215,9 @@ static void rcu_check_quiescent_state(st
struct rcu_state *rsp, struct rcu_data *rdp)
{
if (rdp->quiescbatch != rcp->cur) {
- /* new grace period: record qsctr value. */
+ /* new grace period: reset qsctr value. */
rdp->qs_pending = 1;
- rdp->last_qsctr = rdp->qsctr;
+ rdp->qsctr = 0;
rdp->quiescbatch = rcp->cur;
return;
}
@@ -229,12 +229,7 @@ static void rcu_check_quiescent_state(st
if (!rdp->qs_pending)
return;

- /*
- * Races with local timer interrupt - in the worst case
- * we may miss one quiescent state of that CPU. That is
- * tolerable. So no need to disable interrupts.
- */
- if (rdp->qsctr == rdp->last_qsctr)
+ if (rdp->qsctr == 0)
return;
rdp->qs_pending = 0;