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Subject[PATCH tip/core/rcu 2/3] Need to update rnp->gpnum if preemptable RCU is to be reliable.
Date
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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@linux.vnet.ibm.com>
---
kernel/rcutree.c | 6 +++++-
1 files changed, 5 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 71bc797..03866bd 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -627,6 +627,7 @@ 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;
@@ -652,8 +653,10 @@ 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,
@@ -674,6 +677,7 @@ 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. */
}

--
1.5.2.5


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