rds: introduce acquire/release ordering in acquire/release_in_xmit()

acquire/release_in_xmit() work as bit lock in rds_send_xmit(), so they
are expected to ensure acquire/release memory ordering semantics.
However, test_and_set_bit/clear_bit() don't imply such semantics, on
top of this, following smp_mb__after_atomic() does not guarantee release
ordering (memory barrier actually should be placed before clear_bit()).

Instead, we use clear_bit_unlock/test_and_set_bit_lock() here.

Fixes: 0f4b1c7 ("rds: fix rds_send_xmit() serialization")
Fixes: 1f9ecd7 ("RDS: Pass rds_conn_path to rds_send_xmit()")
Signed-off-by: Yewon Choi <[email protected]>
Reviewed-by: Michal Kubiak <[email protected]>
Link: https://lore.kernel.org/r/ZfQUxnNTO9AJmzwc@libra05
Signed-off-by: Paolo Abeni <[email protected]>

net/rds/send.c

@@ -103,13 +103,12 @@ EXPORT_SYMBOL_GPL(rds_send_path_reset);
 
 static int acquire_in_xmit(struct rds_conn_path *cp)
 {
-	return test_and_set_bit(RDS_IN_XMIT, &cp->cp_flags) == 0;
+	return test_and_set_bit_lock(RDS_IN_XMIT, &cp->cp_flags) == 0;
 }
 
 static void release_in_xmit(struct rds_conn_path *cp)
 {
-	clear_bit(RDS_IN_XMIT, &cp->cp_flags);
-	smp_mb__after_atomic();
+	clear_bit_unlock(RDS_IN_XMIT, &cp->cp_flags);
 	/*
 	 * We don't use wait_on_bit()/wake_up_bit() because our waking is in a
 	 * hot path and finding waiters is very rare.  We don't want to walk