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Subject[PATCH tip/core/rcu 2/2] rcu: fix long-grace-period race between forcing and initialization
Date
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Very long RCU read-side critical sections (50 milliseconds or so) can
cause a race between force_quiescent_state() and rcu_start_gp() as
follows on kernel builds with multi-level rcu_node hierarchies:

1. CPU 0 calls force_quiescent_state(), sees that there is a
grace period in progress, and acquires ->fsqlock.

2. CPU 1 detects the end of the grace period, and so
cpu_quiet_msk_finish() sets rsp->completed to rsp->gpnum.
This operation is carried out under the root rnp->lock,
but CPU 0 has not yet acquired that lock. Note that
rsp->signaled is still RCU_SAVE_DYNTICK from the last
grace period.

3. CPU 1 calls rcu_start_gp(), but no one wants a new grace
period, so it drops the root rnp->lock and returns.

4. CPU 0 acquires the root rnp->lock and picks up rsp->completed
and rsp->signaled, then drops rnp->lock. It then enters the
RCU_SAVE_DYNTICK leg of the switch statement.

5. CPU 2 invokes call_rcu(), and now needs a new grace period.
It calls rcu_start_gp(), which acquires the root rnp->lock, sets
rsp->signaled to RCU_GP_INIT (too bad that CPU 0 is already in
the RCU_SAVE_DYNTICK leg of the switch statement!) and starts
initializing the rcu_node hierarchy. If there are multiple
levels to the hierarchy, it will drop the root rnp->lock and
initialize the lower levels of the hierarchy.

6. CPU 0 notes that rsp->completed has not changed, which permits
both CPU 2 and CPU 0 to try updating it concurrently. If CPU 0's
update prevails, later calls to force_quiescent_state() can
count old quiescent states against the new grace period, which
can in turn result in premature ending of grace periods.

Not good.

This patch adds an RCU_GP_IDLE state for rsp->signaled that is set
initially at boot time and any time a grace period ends. This prevents
CPU 0 from getting into the workings of force_quiescent_state() in
step 4. Additional locking and checks prevent the concurrent update
of rsp->signaled in step 6.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 16 +++++++++++-----
kernel/rcutree.h | 7 ++++---
2 files changed, 15 insertions(+), 8 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 055f1a9..0d9faee 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -61,7 +61,7 @@ static struct lock_class_key rcu_root_class;
NUM_RCU_LVL_2, \
NUM_RCU_LVL_3, /* == MAX_RCU_LVLS */ \
}, \
- .signaled = RCU_SIGNAL_INIT, \
+ .signaled = RCU_GP_IDLE, \
.gpnum = -300, \
.completed = -300, \
.onofflock = __SPIN_LOCK_UNLOCKED(&name.onofflock), \
@@ -659,14 +659,17 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
* irqs disabled.
*/
rcu_for_each_node_breadth_first(rsp, rnp) {
- spin_lock(&rnp->lock); /* irqs already disabled. */
+ spin_lock(&rnp->lock); /* irqs already disabled. */
rcu_preempt_check_blocked_tasks(rnp);
rnp->qsmask = rnp->qsmaskinit;
rnp->gpnum = rsp->gpnum;
- spin_unlock(&rnp->lock); /* irqs already disabled. */
+ spin_unlock(&rnp->lock); /* irqs remain disabled. */
}

+ rnp = rcu_get_root(rsp);
+ spin_lock(&rnp->lock); /* irqs already disabled. */
rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
+ spin_unlock(&rnp->lock); /* irqs remain disabled. */
spin_unlock_irqrestore(&rsp->onofflock, flags);
}

@@ -708,6 +711,7 @@ static void cpu_quiet_msk_finish(struct rcu_state *rsp, unsigned long flags)
{
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
rsp->completed = rsp->gpnum;
+ rsp->signaled = RCU_GP_IDLE;
rcu_process_gp_end(rsp, rsp->rda[smp_processor_id()]);
rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
}
@@ -1151,9 +1155,10 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
}
spin_unlock(&rnp->lock);
switch (signaled) {
+ case RCU_GP_IDLE:
case RCU_GP_INIT:

- break; /* grace period still initializing, ignore. */
+ break; /* grace period idle or initializing, ignore. */

case RCU_SAVE_DYNTICK:

@@ -1167,7 +1172,8 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)

/* Update state, record completion counter. */
spin_lock(&rnp->lock);
- if (lastcomp == rsp->completed) {
+ if (lastcomp == rsp->completed &&
+ rsp->signaled == RCU_SAVE_DYNTICK) {
rsp->signaled = RCU_FORCE_QS;
dyntick_record_completed(rsp, lastcomp);
}
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 599161f..e1bc649 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -201,9 +201,10 @@ struct rcu_data {
};

/* Values for signaled field in struct rcu_state. */
-#define RCU_GP_INIT 0 /* Grace period being initialized. */
-#define RCU_SAVE_DYNTICK 1 /* Need to scan dyntick state. */
-#define RCU_FORCE_QS 2 /* Need to force quiescent state. */
+#define RCU_GP_IDLE 0 /* No grace period in progress. */
+#define RCU_GP_INIT 1 /* Grace period being initialized. */
+#define RCU_SAVE_DYNTICK 2 /* Need to scan dyntick state. */
+#define RCU_FORCE_QS 3 /* Need to force quiescent state. */
#ifdef CONFIG_NO_HZ
#define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
#else /* #ifdef CONFIG_NO_HZ */
--
1.5.2.5


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