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authorJoel Fernandes (Google) <joel@joelfernandes.org>2018-05-22 23:38:13 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2018-07-12 15:39:20 -0700
commit0d805a70a652a6eef8d0283e5183879e7acb85ad (patch)
treea47046bbc2e0808ccfe4e34a33735e76bd40f6fe /kernel/rcu/rcu.h
parentb06ae25a1e2b54b2b5bc589a4a118b7bb39159fe (diff)
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@joelfernandes.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> [ paulmck: Shrink comment as suggested by Peter Zijlstra. ]
Diffstat (limited to 'kernel/rcu/rcu.h')
-rw-r--r--kernel/rcu/rcu.h12
1 files changed, 11 insertions, 1 deletions
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;