Commit 764f6a81 authored by Zilin Guan's avatar Zilin Guan Committed by Boqun Feng
Browse files

rcu: Remove READ_ONCE() for rdp->gpwrap access in __note_gp_changes() 



There is one access to the per-CPU rdp->gpwrap field in the
__note_gp_changes() function that does not use READ_ONCE(), but all other
accesses do use READ_ONCE().  When using the 8*TREE03 and CONFIG_NR_CPUS=8
configuration, KCSAN found no data races at that point.  This is because
all calls to __note_gp_changes() hold rnp->lock, which excludes writes
to the rdp->gpwrap fields for all CPUs associated with that same leaf
rcu_node structure.

This commit therefore removes READ_ONCE() from rdp->gpwrap accesses
within the __note_gp_changes() function.

Signed-off-by: default avatarZilin Guan <zilinguan811@gmail.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
parent a3e81621

kernel/rcu/tree.c

+3 −3
Original line number Diff line number Diff line
@@ -1254,7 +1254,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp) 


	/* Handle the ends of any preceding grace periods first. */

	if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||

	    unlikely(READ_ONCE(rdp->gpwrap))) {

	    unlikely(rdp->gpwrap)) {

		if (!offloaded)

			ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */

		rdp->core_needs_qs = false;
@@ -1268,7 +1268,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp) 


	/* Now handle the beginnings of any new-to-this-CPU grace periods. */

	if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||

	    unlikely(READ_ONCE(rdp->gpwrap))) {

	    unlikely(rdp->gpwrap)) {

		/*

		 * If the current grace period is waiting for this CPU,

		 * set up to detect a quiescent state, otherwise don't
@@ -1283,7 +1283,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp) 
	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */

	if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)

		WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);

	if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap))

	if (IS_ENABLED(CONFIG_PROVE_RCU) && rdp->gpwrap)

		WRITE_ONCE(rdp->last_sched_clock, jiffies);

	WRITE_ONCE(rdp->gpwrap, false);

	rcu_gpnum_ovf(rnp, rdp);