Commit 82980b16 authored by David Woodhouse's avatar David Woodhouse Committed by Paul E. McKenney
Browse files

rcu: Kill rnp->ofl_seq and use only rcu_state.ofl_lock for exclusion 



If we allow architectures to bring APs online in parallel, then we end
up requiring rcu_cpu_starting() to be reentrant. But currently, the
manipulation of rnp->ofl_seq is not thread-safe.

However, rnp->ofl_seq is also fairly much pointless anyway since both
rcu_cpu_starting() and rcu_report_dead() hold rcu_state.ofl_lock for
fairly much the whole time that rnp->ofl_seq is set to an odd number
to indicate that an operation is in progress.

So drop rnp->ofl_seq completely, and use only rcu_state.ofl_lock.

This has a couple of minor complexities: lockdep will complain when we
take rcu_state.ofl_lock, and currently accepts the 'excuse' of having
an odd value in rnp->ofl_seq. So switch it to an arch_spinlock_t to
avoid that false positive complaint. Since we're killing rnp->ofl_seq
of course that 'excuse' has to be changed too, so make it check for
arch_spin_is_locked(rcu_state.ofl_lock).

There's no arch_spin_lock_irqsave() so we have to manually save and
restore local interrupts around the locking.

At Paul's request based on Neeraj's analysis, make rcu_gp_init not just
wait but *exclude* any CPU online/offline activity, which was fairly
much true already by virtue of it holding rcu_state.ofl_lock.

Signed-off-by: default avatarDavid Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent da123016

kernel/rcu/tree.c

+36 −35
Original line number Diff line number Diff line
@@ -91,7 +91,7 @@ static struct rcu_state rcu_state = { 
	.abbr = RCU_ABBR,

	.exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex),

	.exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex),

	.ofl_lock = __RAW_SPIN_LOCK_UNLOCKED(rcu_state.ofl_lock),

	.ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,

};



/* Dump rcu_node combining tree at boot to verify correct setup. */
@@ -1175,7 +1175,15 @@ bool rcu_lockdep_current_cpu_online(void) 
	preempt_disable_notrace();

	rdp = this_cpu_ptr(&rcu_data);

	rnp = rdp->mynode;

	if (rdp->grpmask & rcu_rnp_online_cpus(rnp) || READ_ONCE(rnp->ofl_seq) & 0x1)

	/*

	 * Strictly, we care here about the case where the current CPU is

	 * in rcu_cpu_starting() and thus has an excuse for rdp->grpmask

	 * not being up to date. So arch_spin_is_locked() might have a

	 * false positive if it's held by some *other* CPU, but that's

	 * OK because that just means a false *negative* on the warning.

	 */

	if (rdp->grpmask & rcu_rnp_online_cpus(rnp) ||

	    arch_spin_is_locked(&rcu_state.ofl_lock))

		ret = true;

	preempt_enable_notrace();

	return ret;
@@ -1739,7 +1747,6 @@ static void rcu_strict_gp_boundary(void *unused) 
 */

static noinline_for_stack bool rcu_gp_init(void)

{

	unsigned long firstseq;

	unsigned long flags;

	unsigned long oldmask;

	unsigned long mask;
@@ -1782,22 +1789,17 @@ static noinline_for_stack bool rcu_gp_init(void) 
	 * of RCU's Requirements documentation.

	 */

	WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF);

	/* Exclude CPU hotplug operations. */

	rcu_for_each_leaf_node(rnp) {

		// Wait for CPU-hotplug operations that might have

		// started before this grace period did.

		smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.

		firstseq = READ_ONCE(rnp->ofl_seq);

		if (firstseq & 0x1)

			while (firstseq == READ_ONCE(rnp->ofl_seq))

				schedule_timeout_idle(1);  // Can't wake unless RCU is watching.

		smp_mb(); // Pair with barriers used when updating ->ofl_seq to even values.

		raw_spin_lock(&rcu_state.ofl_lock);

		raw_spin_lock_irq_rcu_node(rnp);

		local_irq_save(flags);

		arch_spin_lock(&rcu_state.ofl_lock);

		raw_spin_lock_rcu_node(rnp);

		if (rnp->qsmaskinit == rnp->qsmaskinitnext &&

		    !rnp->wait_blkd_tasks) {

			/* Nothing to do on this leaf rcu_node structure. */

			raw_spin_unlock_irq_rcu_node(rnp);

			raw_spin_unlock(&rcu_state.ofl_lock);

			raw_spin_unlock_rcu_node(rnp);

			arch_spin_unlock(&rcu_state.ofl_lock);

			local_irq_restore(flags);

			continue;

		}
@@ -1832,8 +1834,9 @@ static noinline_for_stack bool rcu_gp_init(void) 
				rcu_cleanup_dead_rnp(rnp);

		}



		raw_spin_unlock_irq_rcu_node(rnp);

		raw_spin_unlock(&rcu_state.ofl_lock);

		raw_spin_unlock_rcu_node(rnp);

		arch_spin_unlock(&rcu_state.ofl_lock);

		local_irq_restore(flags);

	}

	rcu_gp_slow(gp_preinit_delay); /* Races with CPU hotplug. */
@@ -4287,11 +4290,10 @@ void rcu_cpu_starting(unsigned int cpu) 


	rnp = rdp->mynode;

	mask = rdp->grpmask;

	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);

	WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));

	local_irq_save(flags);

	arch_spin_lock(&rcu_state.ofl_lock);

	rcu_dynticks_eqs_online();

	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().

	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	raw_spin_lock_rcu_node(rnp);

	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);

	newcpu = !(rnp->expmaskinitnext & mask);

	rnp->expmaskinitnext |= mask;
@@ -4304,15 +4306,18 @@ void rcu_cpu_starting(unsigned int cpu) 


	/* An incoming CPU should never be blocking a grace period. */

	if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */

		/* rcu_report_qs_rnp() *really* wants some flags to restore */

		unsigned long flags2;



		local_irq_save(flags2);

		rcu_disable_urgency_upon_qs(rdp);

		/* Report QS -after- changing ->qsmaskinitnext! */

		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);

		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags2);

	} else {

		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

		raw_spin_unlock_rcu_node(rnp);

	}

	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().

	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);

	WARN_ON_ONCE(rnp->ofl_seq & 0x1);

	arch_spin_unlock(&rcu_state.ofl_lock);

	local_irq_restore(flags);

	smp_mb(); /* Ensure RCU read-side usage follows above initialization. */

}
@@ -4326,7 +4331,7 @@ void rcu_cpu_starting(unsigned int cpu) 
 */

void rcu_report_dead(unsigned int cpu)

{

	unsigned long flags;

	unsigned long flags, seq_flags;

	unsigned long mask;

	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

	struct rcu_node *rnp = rdp->mynode;  /* Outgoing CPU's rdp & rnp. */
@@ -4340,10 +4345,8 @@ void rcu_report_dead(unsigned int cpu) 


	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */

	mask = rdp->grpmask;

	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);

	WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));

	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().

	raw_spin_lock(&rcu_state.ofl_lock);

	local_irq_save(seq_flags);

	arch_spin_lock(&rcu_state.ofl_lock);

	raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */

	rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq);

	rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags);
@@ -4354,10 +4357,8 @@ void rcu_report_dead(unsigned int cpu) 
	}

	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);

	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	raw_spin_unlock(&rcu_state.ofl_lock);

	smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().

	WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);

	WARN_ON_ONCE(rnp->ofl_seq & 0x1);

	arch_spin_unlock(&rcu_state.ofl_lock);

	local_irq_restore(seq_flags);



	rdp->cpu_started = false;

}

kernel/rcu/tree.h

+1 −3
Original line number Diff line number Diff line
@@ -56,8 +56,6 @@ struct rcu_node { 
				/*  Initialized from ->qsmaskinitnext at the */

				/*  beginning of each grace period. */

	unsigned long qsmaskinitnext;

	unsigned long ofl_seq;	/* CPU-hotplug operation sequence count. */

				/* Online CPUs for next grace period. */

	unsigned long expmask;	/* CPUs or groups that need to check in */

				/*  to allow the current expedited GP */

				/*  to complete. */
@@ -355,7 +353,7 @@ struct rcu_state { 
	const char *name;			/* Name of structure. */

	char abbr;				/* Abbreviated name. */



	raw_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;

	arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;

						/* Synchronize offline with */

						/*  GP pre-initialization. */

};