linux-kernel - [tip:core/urgent] rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable

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Message-ID: <tip-08ce258a8c263a8b87c652a5661e37b0e7238454@git.kernel.org>
Date:	Sat, 12 Sep 2009 07:55:15 GMT
From:	"tip-bot for Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	hpa@...or.com, mingo@...hat.com, rostedt@...dmis.org,
	tglx@...utronix.de, mingo@...e.hu
Subject: [tip:core/urgent] rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable

Commit-ID:  08ce258a8c263a8b87c652a5661e37b0e7238454
Gitweb:     http://git.kernel.org/tip/08ce258a8c263a8b87c652a5661e37b0e7238454
Author:     Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
AuthorDate: Tue, 8 Sep 2009 15:54:36 -0700
Committer:  Ingo Molnar <mingo@...e.hu>
CommitDate: Sat, 12 Sep 2009 09:47:06 +0200

rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable

Without this patch, tasks preempted in RCU read-side critical
sections can fail to block the grace period, given that
rnp->gpnum is used to determine which rnp->blocked_tasks[]
element the preempted task is enqueued on.

Before the patch, rnp->gpnum is always zero, so preempted tasks
are always enqueued on rnp->blocked_tasks[0], which is correct
only when the current CPU has not checked into the current
grace period and the grace-period number is even, or,
similarly, if the current CPU -has- checked into the current
grace period and the grace-period number is odd.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@...dmis.org>
Cc: laijs@...fujitsu.com
Cc: dipankar@...ibm.com
Cc: akpm@...ux-foundation.org
Cc: mathieu.desnoyers@...ymtl.ca
Cc: josht@...ux.vnet.ibm.com
Cc: dvhltc@...ibm.com
Cc: niv@...ibm.com
Cc: peterz@...radead.org
LKML-Reference: <12524504771622-git-send-email->
Signed-off-by: Ingo Molnar <mingo@...e.hu>


---
 kernel/rcutree.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 6b11b07..c634a92 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -632,6 +632,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	/* Special-case the common single-level case. */
 	if (NUM_RCU_NODES == 1) {
 		rnp->qsmask = rnp->qsmaskinit;
+		rnp->gpnum = rsp->gpnum;
 		rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
 		spin_unlock_irqrestore(&rnp->lock, flags);
 		return;
@@ -657,8 +658,10 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	 */
 
 	rnp_end = rsp->level[NUM_RCU_LVLS - 1];
-	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++)
+	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) {
 		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+		rnp->gpnum = rsp->gpnum;
+	}
 
 	/*
 	 * Now set up the leaf nodes.  Here we must be careful.  First,
@@ -679,6 +682,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	for (; rnp_cur < rnp_end; rnp_cur++) {
 		spin_lock(&rnp_cur->lock);	/* irqs already disabled. */
 		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+		rnp->gpnum = rsp->gpnum;
 		spin_unlock(&rnp_cur->lock);	/* irqs already disabled. */
 	}
 
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