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: cheung wall <zzqq0103.hey@gmail.com>
Closes: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20250227131613.52683-3-urezki@gmail.com
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
index f9bed3d3f78d928547bd5437be592e3c93d3f9ce..4c92d4291cce7acbbd3aed7fc3c655174affe337 100644 (file)
--- a/include/linux/rcupdate_wait.h
+++ b/include/linux/rcupdate_wait.h
@@ -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);
 

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 131fb463ba68ebcc1a9161da701198ec331fabcb..fb98d4d9d9c97ef60087d8a52107eea526dac6f9 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -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);