Hello,
I'm sorry for bothering you, and I seem to be obviously missing
something, but I'm really wondering why we check try_check_zero()
again in the state, SCAN1, for the previous srcu_idx.
I mean, since we've already checked try_check_zero() in the previous
grace period and gotten 'true' as a return value, all readers who see
the flipped idx via srcu_flip() won't update the src_{lock,unlock}_count
for the previous idx until it gets flipped back again.
Is there any reasons we check try_check_zero() again in the state, SCAN1?
Is there any problems if the following patch's applied?
Thanks in advance,
Byungchul
---
kernel/rcu/srcutree.c | 16 +---------------
1 file changed, 1 insertion(+), 15 deletions(-)
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 39e50fe..215c44a 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1125,24 +1125,10 @@ static void srcu_advance_state(struct srcu_struct *sp)
mutex_unlock(&sp->srcu_gp_mutex);
return; /* Someone else started the grace period. */
}
- }
-
- if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
- idx = 1 ^ (sp->srcu_idx & 1);
- if (!try_check_zero(sp, idx, 1)) {
- mutex_unlock(&sp->srcu_gp_mutex);
- return; /* readers present, retry later. */
- }
srcu_flip(sp);
- rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
}
- if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
-
- /*
- * SRCU read-side critical sections are normally short,
- * so check at least twice in quick succession after a flip.
- */
+ if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
idx = 1 ^ (sp->srcu_idx & 1);
if (!try_check_zero(sp, idx, 2)) {
mutex_unlock(&sp->srcu_gp_mutex);
--
1.9.1
