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Date: Fri, 17 Jul 2015 16:29:12 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
bobby.prani@...il.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 07/19] rcu: Abstract sequence counting from synchronize_sched_expedited()
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
This commit creates rcu_exp_gp_seq_start() and rcu_exp_gp_seq_end() to
bracket an expedited grace period, rcu_exp_gp_seq_snap() to snapshot the
sequence counter, and rcu_exp_gp_seq_done() to check to see if a full
expedited grace period has elapsed since the snapshot. These will be
applied to synchronize_rcu_expedited(). These are defined in terms of
underlying rcu_seq_start(), rcu_seq_end(), rcu_seq_snap(), rcu_seq_done(),
which will be applied to _rcu_barrier().
One reason that this commit doesn't use the seqcount primitives themselves
is that the smp_wmb() in those primitive is insufficient due to the fact
that expedited grace periods do reads as well as writes. In addition,
the read-side seqcount primitives detect a potentially partial change,
where the expedited primitives instead need a guaranteed full change.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
kernel/rcu/tree.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 58 insertions(+), 10 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c5c8509054ef..67fe75725486 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3255,6 +3255,60 @@ void cond_synchronize_rcu(unsigned long oldstate)
}
EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
+/* Adjust sequence number for start of update-side operation. */
+static void rcu_seq_start(unsigned long *sp)
+{
+ WRITE_ONCE(*sp, *sp + 1);
+ smp_mb(); /* Ensure update-side operation after counter increment. */
+ WARN_ON_ONCE(!(*sp & 0x1));
+}
+
+/* Adjust sequence number for end of update-side operation. */
+static void rcu_seq_end(unsigned long *sp)
+{
+ smp_mb(); /* Ensure update-side operation before counter increment. */
+ WRITE_ONCE(*sp, *sp + 1);
+ WARN_ON_ONCE(*sp & 0x1);
+}
+
+/* Take a snapshot of the update side's sequence number. */
+static unsigned long rcu_seq_snap(unsigned long *sp)
+{
+ unsigned long s;
+
+ smp_mb(); /* Caller's modifications seen first by other CPUs. */
+ s = (READ_ONCE(*sp) + 3) & ~0x1;
+ smp_mb(); /* Above access must not bleed into critical section. */
+ return s;
+}
+
+/*
+ * Given a snapshot from rcu_seq_snap(), determine whether or not a
+ * full update-side operation has occurred.
+ */
+static bool rcu_seq_done(unsigned long *sp, unsigned long s)
+{
+ return ULONG_CMP_GE(READ_ONCE(*sp), s);
+}
+
+/* Wrapper functions for expedited grace periods. */
+static void rcu_exp_gp_seq_start(struct rcu_state *rsp)
+{
+ rcu_seq_start(&rsp->expedited_sequence);
+}
+static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
+{
+ rcu_seq_end(&rsp->expedited_sequence);
+}
+static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
+{
+ return rcu_seq_snap(&rsp->expedited_sequence);
+}
+static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
+{
+ return rcu_seq_done(&rsp->expedited_sequence, s);
+}
+
static int synchronize_sched_expedited_cpu_stop(void *data)
{
struct rcu_state *rsp = data;
@@ -3269,7 +3323,7 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
static bool sync_sched_exp_wd(struct rcu_state *rsp, struct rcu_node *rnp,
atomic_long_t *stat, unsigned long s)
{
- if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) {
+ if (rcu_exp_gp_seq_done(rsp, s)) {
if (rnp)
mutex_unlock(&rnp->exp_funnel_mutex);
/* Ensure test happens before caller kfree(). */
@@ -3306,9 +3360,7 @@ void synchronize_sched_expedited(void)
struct rcu_state *rsp = &rcu_sched_state;
/* Take a snapshot of the sequence number. */
- smp_mb(); /* Caller's modifications seen first by other CPUs. */
- s = (READ_ONCE(rsp->expedited_sequence) + 3) & ~0x1;
- smp_mb(); /* Above access must not bleed into critical section. */
+ s = rcu_exp_gp_seq_snap(rsp);
if (!try_get_online_cpus()) {
/* CPU hotplug operation in flight, fall back to normal GP. */
@@ -3339,9 +3391,7 @@ void synchronize_sched_expedited(void)
if (sync_sched_exp_wd(rsp, rnp0, &rsp->expedited_workdone2, s))
return;
- WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1);
- smp_mb(); /* Ensure expedited GP seen after counter increment. */
- WARN_ON_ONCE(!(rsp->expedited_sequence & 0x1));
+ rcu_exp_gp_seq_start(rsp);
/* Stop each CPU that is online, non-idle, and not us. */
init_waitqueue_head(&rsp->expedited_wq);
@@ -3364,9 +3414,7 @@ void synchronize_sched_expedited(void)
wait_event(rsp->expedited_wq,
!atomic_read(&rsp->expedited_need_qs));
- smp_mb(); /* Ensure expedited GP seen before counter increment. */
- WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1);
- WARN_ON_ONCE(rsp->expedited_sequence & 0x1);
+ rcu_exp_gp_seq_end(rsp);
mutex_unlock(&rnp0->exp_funnel_mutex);
smp_mb(); /* ensure subsequent action seen after grace period. */
--
1.8.1.5
--
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