Commit 5a562b8b authored by Uladzislau Rezki (Sony)'s avatar Uladzislau Rezki (Sony) Committed by Boqun Feng
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rcu: Use _full() API to debug synchronize_rcu() 



Switch for using of get_state_synchronize_rcu_full() and
poll_state_synchronize_rcu_full() pair to debug a normal
synchronize_rcu() call.

Just using "not" full APIs to identify if a grace period is
passed or not might lead to a false-positive kernel splat.

It can happen, because get_state_synchronize_rcu() compresses
both normal and expedited states into one single unsigned long
value, so a poll_state_synchronize_rcu() can miss GP-completion
when synchronize_rcu()/synchronize_rcu_expedited() concurrently
run.

To address this, switch to poll_state_synchronize_rcu_full() and
get_state_synchronize_rcu_full() APIs, which use separate variables
for expedited and normal states.

Reported-by: default avatarcheung wall <zzqq0103.hey@gmail.com>
Closes: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/


Fixes: 988f569a ("rcu: Reduce synchronize_rcu() latency")
Signed-off-by: default avatarUladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20250227131613.52683-3-urezki@gmail.com


Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
parent a6cea395

include/linux/rcupdate_wait.h

+3 −0
Original line number Diff line number Diff line
@@ -16,6 +16,9 @@ 
struct rcu_synchronize {

	struct rcu_head head;

	struct completion completion;



	/* This is for debugging. */

	struct rcu_gp_oldstate oldstate;

};

void wakeme_after_rcu(struct rcu_head *head);

kernel/rcu/tree.c

+3 −5
Original line number Diff line number Diff line
@@ -1612,12 +1612,10 @@ static void rcu_sr_normal_complete(struct llist_node *node) 
{

	struct rcu_synchronize *rs = container_of(

		(struct rcu_head *) node, struct rcu_synchronize, head);

	unsigned long oldstate = (unsigned long) rs->head.func;



	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&

		!poll_state_synchronize_rcu(oldstate),

		"A full grace period is not passed yet: %lu",

		rcu_seq_diff(get_state_synchronize_rcu(), oldstate));

		!poll_state_synchronize_rcu_full(&rs->oldstate),

		"A full grace period is not passed yet!\n");



	/* Finally. */

	complete(&rs->completion);
@@ -3218,7 +3216,7 @@ static void synchronize_rcu_normal(void) 
	 * snapshot before adding a request.

	 */

	if (IS_ENABLED(CONFIG_PROVE_RCU))

		rs.head.func = (void *) get_state_synchronize_rcu();

		get_state_synchronize_rcu_full(&rs.oldstate);



	rcu_sr_normal_add_req(&rs);