[RFC PATCH 04/10] rcu: Add RCU_GET_STATE_NOT_TRACKED for subsystems without expedited GPs

[Puranjay Mohan <puranjay@kernel.org>]

SRCU and Tasks RCU do not track expedited grace periods. When their
callback state is checked via poll_state_synchronize_rcu_full(), the
uninitialized or zeroed rgos_exp field could cause false-positive
completion detection.

This commit adds an RCU_GET_STATE_NOT_TRACKED sentinel value (0x2) that
these subsystems can place into rgos_exp to indicate that expedited GP
tracking is not applicable. The expedited sequence check in
poll_state_synchronize_rcu_full() is guarded to skip entries marked with
this sentinel.

This is needed to allow rcu_segcblist_advance() and rcu_accelerate_cbs()
to work with both normal and expedited grace periods via
get_state_synchronize_rcu_full() and poll_state_synchronize_rcu_full().

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/rcu/rcu.h  | 13 +++++++++++--
 kernel/rcu/tree.c |  3 ++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index fa6d30ce73d1..b450febac823 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -46,16 +46,25 @@
  *					the number of pending readers that will use
  *					this inactive index is bounded).
  *
- * RCU polled GP special control value:
+ * RCU polled GP special control values:
  *
  *	RCU_GET_STATE_COMPLETED :	State value indicating an already-completed
  *					polled GP has completed.  This value covers
  *					both the state and the counter of the
  *					grace-period sequence number.
+ *
+ *	RCU_GET_STATE_NOT_TRACKED :	State value indicating that a GP component
+ *					is not tracked by this subsystem and should
+ *					not be checked.  Used by SRCU and RCU Tasks
+ *					which do not track expedited GPs, to prevent
+ *					false-positive completion when their
+ *					gp_seq_full entries are checked via
+ *					poll_state_synchronize_rcu_full().
  */
 
-/* Low-order bit definition for polled grace-period APIs. */
+/* Low-order bit definitions for polled grace-period APIs. */
 #define RCU_GET_STATE_COMPLETED	0x1
+#define RCU_GET_STATE_NOT_TRACKED	0x2
 
 /* A complete grace period count */
 #define RCU_SEQ_GP (RCU_SEQ_STATE_MASK + 1)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cbc170dc3f72..607fc5715cd1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3603,7 +3603,8 @@ bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
 	if (rgosp->rgos_norm == RCU_GET_STATE_COMPLETED ||
 	    rcu_seq_done_exact(&rnp->gp_seq, rgosp->rgos_norm) ||
 	    rgosp->rgos_exp == RCU_GET_STATE_COMPLETED ||
-	    rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp)) {
+	    (rgosp->rgos_exp != RCU_GET_STATE_NOT_TRACKED &&
+	     rcu_seq_done_exact(&rcu_state.expedited_sequence, rgosp->rgos_exp))) {
 		smp_mb(); /* Ensure GP ends before subsequent accesses. */
 		return true;
 	}
-- 
2.52.0