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-rw-r--r--kernel/rcu/srcutree.c59
1 files changed, 58 insertions, 1 deletions
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 4b98e6f45166..2286e06fd159 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -402,6 +402,7 @@ static void srcu_gp_start(struct srcu_struct *sp)
rcu_seq_current(&sp->srcu_gp_seq));
(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
rcu_seq_snap(&sp->srcu_gp_seq));
+ smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
rcu_seq_start(&sp->srcu_gp_seq);
state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
@@ -677,6 +678,57 @@ static void srcu_flip(struct srcu_struct *sp)
}
/*
+ * If SRCU is likely idle, return true, otherwise return false.
+ *
+ * Note that it is OK for several current from-idle requests for a new
+ * grace period from idle to specify expediting because they will all end
+ * up requesting the same grace period anyhow. So no loss.
+ *
+ * Note also that if any CPU (including the current one) is still invoking
+ * callbacks, this function will nevertheless say "idle". This is not
+ * ideal, but the overhead of checking all CPUs' callback lists is even
+ * less ideal, especially on large systems. Furthermore, the wakeup
+ * can happen before the callback is fully removed, so we have no choice
+ * but to accept this type of error.
+ *
+ * This function is also subject to counter-wrap errors, but let's face
+ * it, if this function was preempted for enough time for the counters
+ * to wrap, it really doesn't matter whether or not we expedite the grace
+ * period. The extra overhead of a needlessly expedited grace period is
+ * negligible when amoritized over that time period, and the extra latency
+ * of a needlessly non-expedited grace period is similarly negligible.
+ */
+static bool srcu_might_be_idle(struct srcu_struct *sp)
+{
+ unsigned long flags;
+ struct srcu_data *sdp;
+ unsigned long curseq;
+
+ /* If the local srcu_data structure has callbacks, not idle. */
+ local_irq_save(flags);
+ sdp = this_cpu_ptr(sp->sda);
+ if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
+ local_irq_restore(flags);
+ return false; /* Callbacks already present, so not idle. */
+ }
+ local_irq_restore(flags);
+
+ /*
+ * No local callbacks, so probabalistically probe global state.
+ * Exact information would require acquiring locks, which would
+ * kill scalability, hence the probabalistic nature of the probe.
+ */
+ curseq = rcu_seq_current(&sp->srcu_gp_seq);
+ smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
+ if (ULONG_CMP_LT(curseq, READ_ONCE(sp->srcu_gp_seq_needed)))
+ return false; /* Grace period in progress, so not idle. */
+ smp_mb(); /* Order ->srcu_gp_seq with prior access. */
+ if (curseq != rcu_seq_current(&sp->srcu_gp_seq))
+ return false; /* GP # changed, so not idle. */
+ return true; /* With reasonable probability, idle! */
+}
+
+/*
* Enqueue an SRCU callback on the srcu_data structure associated with
* the current CPU and the specified srcu_struct structure, initiating
* grace-period processing if it is not already running.
@@ -823,10 +875,15 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
* Of course, these memory-ordering guarantees apply only when
* synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
* passed the same srcu_struct structure.
+ *
+ * If SRCU is likely idle, expedite the first request. This semantic
+ * was provided by Classic SRCU, and is relied upon by its users, so TREE
+ * SRCU must also provide it. Note that detecting idleness is heuristic
+ * and subject to both false positives and negatives.
*/
void synchronize_srcu(struct srcu_struct *sp)
{
- if (rcu_gp_is_expedited())
+ if (srcu_might_be_idle(sp) || rcu_gp_is_expedited())
synchronize_srcu_expedited(sp);
else
__synchronize_srcu(sp, true);