The rcu_seq_done() API has a large "false-negative" windows of size
ULONG_MAX/2, where after wrap around, it is possible that it will think
that a GP has not completed if a wrap around happens and the delta is
large.
rcu_seq_done_exact() is more accurate avoiding this wrap around issue,
by making the window of false-negativity by only 3 GPs. Use this logic
for rcu_seq_done() which is a nice negative code delta and could
potentially avoid issues in the future where rcu_seq_done() was
reporting false-negatives for too long.
rcutorture runs of all scenarios for 15 minutes passed. Code inspection
was done of all users to convince the change would work.
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/rcu.h | 13 ++-----------
kernel/rcu/tree.c | 6 +++---
2 files changed, 5 insertions(+), 14 deletions(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index eed2951a4962..c2ca196907cb 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -146,19 +146,10 @@ static inline bool rcu_seq_started(unsigned long *sp, unsigned long s)
/*
* Given a snapshot from rcu_seq_snap(), determine whether or not a
- * full update-side operation has occurred.
+ * full update-side operation has occurred while also handling
+ * wraparounds that exceed the (ULONG_MAX / 2) safety-factor/guard-band.
*/
static inline bool rcu_seq_done(unsigned long *sp, unsigned long s)
-{
- return ULONG_CMP_GE(READ_ONCE(*sp), s);
-}
-
-/*
- * Given a snapshot from rcu_seq_snap(), determine whether or not a
- * full update-side operation has occurred, but do not allow the
- * (ULONG_MAX / 2) safety-factor/guard-band.
- */
-static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
{
unsigned long cur_s = READ_ONCE(*sp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b77ccc55557b..835600cec9ba 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4300,7 +4300,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
bool poll_state_synchronize_rcu(unsigned long oldstate)
{
if (oldstate == RCU_GET_STATE_COMPLETED ||
- rcu_seq_done_exact(&rcu_state.gp_seq_polled, oldstate)) {
+ rcu_seq_done(&rcu_state.gp_seq_polled, oldstate)) {
smp_mb(); /* Ensure GP ends before subsequent accesses. */
return true;
}
@@ -4347,9 +4347,9 @@ bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
smp_mb(); // Order against root rcu_node structure grace-period cleanup.
if (rgosp->rgos_norm == RCU_GET_STATE_COMPLETED ||
- rcu_seq_done_exact(&rnp->gp_seq, rgosp->rgos_norm) ||
+ rcu_seq_done(&rnp->gp_seq, rgosp->rgos_norm) ||
rgosp->rgos_exp == RCU_GET_STATE_COMPLETED ||
- rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
+ rcu_seq_done(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
smp_mb(); /* Ensure GP ends before subsequent accesses. */
return true;
}
--
2.34.1
