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Commit 2ee5aca5 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Make rcu_seq_diff() more exact 



The current implementatation of rcu_seq_diff() follows tradition in
providing a rough-and-ready approximation of the number of elapsed grace
periods between the two rcu_seq values.  However, this difference is
used to flag RCU-failure "near misses", which can be a valuable debugging
aid, so more exactitude would be an improvement.  This commit therefore
improves the accuracy of rcu_seq_diff().

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 264d4f88

kernel/rcu/rcu.h

+14 −1
Original line number Diff line number Diff line
@@ -158,7 +158,20 @@ static inline bool rcu_seq_new_gp(unsigned long old, unsigned long new) 
 */

static inline unsigned long rcu_seq_diff(unsigned long new, unsigned long old)

{

	return (new - old) >> RCU_SEQ_CTR_SHIFT;

	unsigned long rnd_diff;



	if (old == new)

		return 0;

	/*

	 * Compute the number of grace periods (still shifted up), plus

	 * one if either of new and old is not an exact grace period.

	 */

	rnd_diff = (new & ~RCU_SEQ_STATE_MASK) -

		   ((old + RCU_SEQ_STATE_MASK) & ~RCU_SEQ_STATE_MASK) +

		   ((new & RCU_SEQ_STATE_MASK) || (old & RCU_SEQ_STATE_MASK));

	if (ULONG_CMP_GE(RCU_SEQ_STATE_MASK, rnd_diff))

		return 1; /* Definitely no grace period has elapsed. */

	return ((rnd_diff - RCU_SEQ_STATE_MASK - 1) >> RCU_SEQ_CTR_SHIFT) + 2;

}



/*