Commit 83f5b01f authored Oct 28, 2009 by Paul E. McKenney Committed by Ingo Molnar Nov 02, 2009

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>

parent b00bc0b2

kernel/rcutree.c

+11 −5

Original line number	Diff line number	Diff line
		@@ -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), \
		@@ -661,10 +661,13 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
		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);
		}

kernel/rcutree.h

+4 −3

Original line number	Diff line number	Diff line
		@@ -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 */