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, 3 insertions, 0 deletions

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 0adb55941aeb..32291f4eefde 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -23,6 +23,9 @@
 #define RCU_SEQ_CTR_SHIFT	2
 #define RCU_SEQ_STATE_MASK	((1 << RCU_SEQ_CTR_SHIFT) - 1)
 
+/* Low-order bit definition for polled grace-period APIs. */
+#define RCU_GET_STATE_COMPLETED	0x1
+
 extern int sysctl_sched_rt_runtime;
 
 /*