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Commit de078d87 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Ingo Molnar
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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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
LKML-Reference: <12524504771622-git-send-email->
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent de55a895

kernel/rcutree.c

+5 −1
Original line number Diff line number Diff line
@@ -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. */

	}