[PATCH 2/6] rcu/nocb: Remove needless full barrier after callback advancing

Boqun Feng
2024-02-01T01:42:14Z

From: Frederic Weisbecker <frederic@kernel.org>

A full barrier is issued from nocb_gp_wait() upon callbacks advancing
to order grace period completion with callbacks execution.

However these two events are already ordered by the
smp_mb__after_unlock_lock() barrier within the call to
raw_spin_lock_rcu_node() that is necessary for callbacks advancing to
happen.

The following litmus test shows the kind of guarantee that this barrier
provides:

C smp_mb__after_unlock_lock

{}

// rcu_gp_cleanup()
P0(spinlock_t *rnp_lock, int *gpnum)
{
// Grace period cleanup increase gp sequence number
spin_lock(rnp_lock);
WRITE_ONCE(*gpnum, 1);
spin_unlock(rnp_lock);
}

// nocb_gp_wait()
P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready)
{
int r1;

// Call rcu_advance_cbs() from nocb_gp_wait()
spin_lock(nocb_lock);
spin_lock(rnp_lock);
smp_mb__after_unlock_lock();
r1 = READ_ONCE(*gpnum);
WRITE_ONCE(*cb_ready, 1);
spin_unlock(rnp_lock);
spin_unlock(nocb_lock);
}

// nocb_cb_wait()
P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed)
{
int r2;

// rcu_do_batch() -> rcu_segcblist_extract_done_cbs()
spin_lock(nocb_lock);
r2 = READ_ONCE(*cb_ready);
spin_unlock(nocb_lock);

// Actual callback execution
WRITE_ONCE(*cb_executed, 1);
}

P3(int *cb_executed, int *gpnum)
{
int r3;

WRITE_ONCE(*cb_executed, 2);
smp_mb();
r3 = READ_ONCE(*gpnum);
}

exists (1:r1=1 /\ 2:r2=1 /\ cb_executed=2 /\ 3:r3=0) (* Bad outcome. *)

Here the bad outcome only occurs if the smp_mb__after_unlock_lock() is
removed. This barrier orders the grace period completion against
callbacks advancing and even later callbacks invocation, thanks to the
opportunistic propagation via the ->nocb_lock to nocb_cb_wait().

Therefore the smp_mb() placed after callbacks advancing can be safely
removed.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
kernel/rcu/tree.c | 6 ++++++
kernel/rcu/tree_nocb.h | 1 -
2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b2bccfd37c38..d540d210e5c7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2145,6 +2145,12 @@ static void rcu_do_batch(struct rcu_data *rdp)
* Extract the list of ready callbacks, disabling IRQs to prevent
* races with call_rcu() from interrupt handlers. Leave the
* callback counts, as rcu_barrier() needs to be conservative.
+ *
+ * Callbacks execution is fully ordered against preceding grace period
+ * completion (materialized by rnp->gp_seq update) thanks to the
+ * smp_mb__after_unlock_lock() upon node locking required for callbacks
+ * advancing. In NOCB mode this ordering is then further relayed through
+ * the nocb locking that protects both callbacks advancing and extraction.
*/
rcu_nocb_lock_irqsave(rdp, flags);
WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 785946834c6b..b2c3145c4c13 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -779,7 +779,6 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
needwake = rdp->nocb_cb_sleep;
WRITE_ONCE(rdp->nocb_cb_sleep, false);
- smp_mb(); /* CB invocation -after- GP end. */
} else {
needwake = false;
}
--
2.43.0