rcu: Fix long-grace-period race between forcing and initialization

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>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1256742889199-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 15 insertions, 8 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 0536125b0497..f3077c0ab181 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -59,7 +59,7 @@
                 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), \
@@ -657,14 +657,17 @@ 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);
 }
 
@@ -706,6 +709,7 @@ 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. */
 }
@@ -1162,9 +1166,10 @@ 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:
 
@@ -1178,7 +1183,8 @@ 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 1823c6e20609..1899023b0962 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 */