rcu: Make rcu_seq_diff() more exact (2ee5aca5) · Commits · 戴 / test

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:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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;
		}

		/*

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