rcutorture: Correctly handle grace-period sequence wrap

The new ->gq_seq grace-period sequence numbers must be shifted down,
which give artifacts when these numbers wrap.  This commit therefore
enables rcutorture and rcuperf to handle grace-period sequence numbers
even if they do wrap.  It does this by allowing a special subtraction
function to be specified, and this function subtracts before shifting.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 1c5cbd9d7c97..aa215d6355f8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -143,6 +143,15 @@ static inline bool rcu_seq_new_gp(unsigned long old, unsigned long new)
 }
 
 /*
+ * Roughly how many full grace periods have elapsed between the collection
+ * of the two specified grace periods?
+ */
+static inline unsigned long rcu_seq_diff(unsigned long new, unsigned long old)
+{
+	return (new - old) >> RCU_SEQ_CTR_SHIFT;
+}
+
+/*
  * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
  * by call_rcu() and rcu callback execution, and are therefore not part of the
  * RCU API. Leaving in rcupdate.h because they are used by all RCU flavors.