From: Joel Fernandes (Google) Date: Wed, 23 May 2018 06:38:13 +0000 (-0700) Subject: rcu: Add comment documenting how rcu_seq_snap works X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=0d805a70a652a6eef8d0283e5183879e7acb85ad;p=linux.git rcu: Add comment documenting how rcu_seq_snap works rcu_seq_snap may be tricky to decipher. Lets document how it works with an example to make it easier. Signed-off-by: Joel Fernandes (Google) Signed-off-by: Paul E. McKenney [ paulmck: Shrink comment as suggested by Peter Zijlstra. ] --- diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index aa215d6355f87..89f13fffac73a 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -91,7 +91,17 @@ static inline void rcu_seq_end(unsigned long *sp) WRITE_ONCE(*sp, rcu_seq_endval(sp)); } -/* Take a snapshot of the update side's sequence number. */ +/* + * rcu_seq_snap - Take a snapshot of the update side's sequence number. + * + * This function returns the earliest value of the grace-period sequence number + * that will indicate that a full grace period has elapsed since the current + * time. Once the grace-period sequence number has reached this value, it will + * be safe to invoke all callbacks that have been registered prior to the + * current time. This value is the current grace-period number plus two to the + * power of the number of low-order bits reserved for state, then rounded up to + * the next value in which the state bits are all zero. + */ static inline unsigned long rcu_seq_snap(unsigned long *sp) { unsigned long s;