rcu: Replace magic number with meaningful constant in rcu_seq_done_exact()

The rcu_seq_done_exact() function checks if a grace period has completed by
comparing sequence numbers. It includes a guard band to handle sequence number
wraparound, which was previously expressed using the magic number calculation
'3 * RCU_SEQ_STATE_MASK + 1'.

This magic number is not immediately obvious in terms of what it represents.

Instead, the reason we need this tiny guardband is because of the lag between
the setting of rcu_state.gp_seq_polled and root rnp's gp_seq in rcu_gp_init().

This guardband needs to be at least 2 GPs worth of counts, to avoid recognizing
the newly started GP as completed immediately, due to the following sequence
which arises due to the delay between update of rcu_state.gp_seq_polled and
root rnp's gp_seq:

rnp->gp_seq = rcu_state.gp_seq = 0

    CPU 0                                           CPU 1
    -----                                           -----
    // rcu_state.gp_seq = 1
    rcu_seq_start(&rcu_state.gp_seq)
                                                    // snap = 8
                                                    snap = rcu_seq_snap(&rcu_state.gp_seq)
                                                    // Two full GP differences
                                                    rcu_seq_done_exact(&rnp->gp_seq, snap)
    // rnp->gp_seq = 1
    WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq);

This can happen due to get_state_synchronize_rcu_full() sampling
rcu_state.gp_seq_polled, however the poll_state_synchronize_rcu_full()
sampling the root rnp's gp_seq. The delay between the update of the 2
counters occurs in rcu_gp_init() during which the counters briefly go
out of sync.

Make the guardband explictly 2 GPs. This improves code readability and
maintainability by making the intent clearer as well.

Suggested-by: Frederic Weisbecker <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Signed-off-by: Joel Fernandes <[email protected]>

Author: Joel Fernandes

kernel/rcu/rcu.h

@@ -57,6 +57,9 @@
 /* Low-order bit definition for polled grace-period APIs. */
 #define RCU_GET_STATE_COMPLETED	0x1
 
+/* A complete grace period count */
+#define RCU_SEQ_GP (RCU_SEQ_STATE_MASK + 1)
+
 extern int sysctl_sched_rt_runtime;
 
 /*
@@ -162,7 +165,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s)
 {
 	unsigned long cur_s = READ_ONCE(*sp);
 
-	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1));
+	return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_GP));
 }
 
 /*