This commit emphasizes the possibility of concurrent calls to synchronize_rcu() and synchronize_rcu_expedited() causing one or the other of the two grace periods being lost from the viewpoint of poll_state_synchronize_rcu(). If you cannot afford to lose grace periods this way, you should instead use the _full() variants of the polled RCU API, for example, poll_state_synchronize_rcu_full(). Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tree.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 7d3a59d4f37ef..0147e69ea85a9 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3559,7 +3559,9 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full); * If @false is returned, it is the caller's responsibility to invoke this * function later on until it does return @true. Alternatively, the caller * can explicitly wait for a grace period, for example, by passing @oldstate - * to cond_synchronize_rcu() or by directly invoking synchronize_rcu(). + * to either cond_synchronize_rcu() or cond_synchronize_rcu_expedited() + * on the one hand or by directly invoking either synchronize_rcu() or + * synchronize_rcu_expedited() on the other. * * Yes, this function does not take counter wrap into account. * But counter wrap is harmless. If the counter wraps, we have waited for @@ -3570,6 +3572,12 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full); * completed. Alternatively, they can use get_completed_synchronize_rcu() * to get a guaranteed-completed grace-period state. * + * In addition, because oldstate compresses the grace-period state for + * both normal and expedited grace periods into a single unsigned long, + * it can miss a grace period when synchronize_rcu() runs concurrently + * with synchronize_rcu_expedited(). If this is unacceptable, please + * instead use the _full() variant of these polling APIs. + * * This function provides the same memory-ordering guarantees that * would be provided by a synchronize_rcu() that was invoked at the call * to the function that provided @oldstate, and that returned at the end -- 2.31.1.189.g2e36527f23
