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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-06-09 10:22:20 +0200 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2018-07-13 00:39:23 +0200 |
| commit | 2ee5aca54622aacc196106c623fea4116f1043a6 (patch) | |
| tree | f364e3a66f8715e2b39035a4d86724f27e3199b0 /kernel/rcu/rcu.h | |
| parent | doc: Update synchronize_rcu() definition in whatisRCU.txt (diff) | |
| download | linux-2ee5aca54622aacc196106c623fea4116f1043a6.tar.xz linux-2ee5aca54622aacc196106c623fea4116f1043a6.zip | |
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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcu/rcu.h')
| -rw-r--r-- | kernel/rcu/rcu.h | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 89f13fffac73..d5e0294b8580 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -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; } /* |