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Subject[PATCH tip/core/rcu 33/40] srcu: Improve rcu_seq grace-period-counter abstraction
Date
The expedited grace-period code contains several open-coded shifts
know the format of an rcu_seq grace-period counter, which is not
particularly good style. This commit therefore creates a new
rcu_seq_ctr() function that extracts the counter portion of the
counter, and an rcu_seq_state() function that extracts the low-order
state bit. This commit prepares for SRCU callback parallelization,
which will require two state bits.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/rcu.h | 29 +++++++++++++++++++++++++----
kernel/rcu/tree_exp.h | 9 ++++-----
2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 4303b880ac99..c62df93bfc1b 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -61,20 +61,41 @@
* Grace-period counter management.
*/

+#define RCU_SEQ_CTR_SHIFT 1
+#define RCU_SEQ_STATE_MASK ((1 << RCU_SEQ_CTR_SHIFT) - 1)
+
+/*
+ * Return the counter portion of a sequence number previously returned
+ * by rcu_seq_snap() or rcu_seq_current().
+ */
+static inline unsigned long rcu_seq_ctr(unsigned long s)
+{
+ return s >> RCU_SEQ_CTR_SHIFT;
+}
+
+/*
+ * Return the state portion of a sequence number previously returned
+ * by rcu_seq_snap() or rcu_seq_current().
+ */
+static inline int rcu_seq_state(unsigned long s)
+{
+ return s & RCU_SEQ_STATE_MASK;
+}
+
/* Adjust sequence number for start of update-side operation. */
static inline 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));
+ WARN_ON_ONCE(rcu_seq_state(*sp) != 1);
}

/* Adjust sequence number for end of update-side operation. */
static inline void rcu_seq_end(unsigned long *sp)
{
smp_mb(); /* Ensure update-side operation before counter increment. */
- WARN_ON_ONCE(!(*sp & 0x1));
- WRITE_ONCE(*sp, *sp + 1);
+ WARN_ON_ONCE(!rcu_seq_state(*sp));
+ WRITE_ONCE(*sp, (*sp | RCU_SEQ_STATE_MASK) + 1);
}

/* Take a snapshot of the update side's sequence number. */
@@ -82,7 +103,7 @@ static inline unsigned long rcu_seq_snap(unsigned long *sp)
{
unsigned long s;

- s = (READ_ONCE(*sp) + 3) & ~0x1;
+ s = (READ_ONCE(*sp) + 2 * RCU_SEQ_STATE_MASK + 1) & ~RCU_SEQ_STATE_MASK;
smp_mb(); /* Above access must not bleed into critical section. */
return s;
}
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 027e123d93c7..e513b4ab1197 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -292,7 +292,7 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
trace_rcu_exp_funnel_lock(rsp->name, rnp->level,
rnp->grplo, rnp->grphi,
TPS("wait"));
- wait_event(rnp->exp_wq[(s >> 1) & 0x3],
+ wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
sync_exp_work_done(rsp,
&rdp->exp_workdone2, s));
return true;
@@ -534,7 +534,7 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
spin_unlock(&rnp->exp_lock);
}
smp_mb(); /* All above changes before wakeup. */
- wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]);
+ wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]);
}
trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake"));
mutex_unlock(&rsp->exp_wake_mutex);
@@ -612,9 +612,8 @@ static void _synchronize_rcu_expedited(struct rcu_state *rsp,
/* Wait for expedited grace period to complete. */
rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
rnp = rcu_get_root(rsp);
- wait_event(rnp->exp_wq[(s >> 1) & 0x3],
- sync_exp_work_done(rsp,
- &rdp->exp_workdone0, s));
+ wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
+ sync_exp_work_done(rsp, &rdp->exp_workdone0, s));
smp_mb(); /* Workqueue actions happen before return. */

/* Let the next expedited grace period start. */
--
2.5.2
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