rcu: Provide a get_completed_synchronize_rcu() function

It is currently up to the caller to handle stale return values from
get_state_synchronize_rcu().  If poll_state_synchronize_rcu() returned
true once, a grace period has elapsed, regardless of the fact that counter
wrap might cause some future poll_state_synchronize_rcu() invocation to
return false.  For example, the caller might store a separate flag that
indicates whether some previous call to poll_state_synchronize_rcu()
determined that the relevant grace period had already ended.

This approach works, but it requires extra storage and is easy to get
wrong.  This commit therefore introduces a get_completed_synchronize_rcu()
that returns a cookie that causes poll_state_synchronize_rcu() to always
return true.  This already-completed cookie can be stored in place of the
cookie that previously caused poll_state_synchronize_rcu() to return true.
It can also be used to flag a given structure as not having been exposed
to readers, and thus not requiring a grace period to elapse.

This commit is in preparation for polled expedited grace periods.

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index fc7fef575606..2e93acad1e31 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -516,6 +516,19 @@ int rcu_cpu_stall_suppress_at_boot __read_mostly; // !0 = suppress boot stalls.
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress_at_boot);
 module_param(rcu_cpu_stall_suppress_at_boot, int, 0444);
 
+/**
+ * get_completed_synchronize_rcu - Return a pre-completed polled state cookie
+ *
+ * Returns a value that will always be treated by functions like
+ * poll_state_synchronize_rcu() as a cookie whose grace period has already
+ * completed.
+ */
+unsigned long get_completed_synchronize_rcu(void)
+{
+	return RCU_GET_STATE_COMPLETED;
+}
+EXPORT_SYMBOL_GPL(get_completed_synchronize_rcu);
+
 #ifdef CONFIG_PROVE_RCU
 
 /*