Commit-ID: 0d805a70a652a6eef8d0283e5183879e7acb85ad
Gitweb: https://git.kernel.org/tip/0d805a70a652a6eef8d0283e5183879e7acb85ad
Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 22 May 2018 23:38:13 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Thu, 12 Jul 2018 15:39:20 -0700
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) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
[ paulmck: Shrink comment as suggested by Peter Zijlstra. ]
---
kernel/rcu/rcu.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index aa215d6355f8..89f13fffac73 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;
--
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