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rcu: Make rcu_seq_diff() more exact

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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>
This commit is contained in:
Paul E. McKenney 2018-06-09 01:22:20 -07:00
parent 264d4f88ad
commit 2ee5aca546
1 changed files with 14 additions and 1 deletions
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15
kernel/rcu/rcu.h
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@ -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;
}
/*