srcu: Improve rcu_seq grace-period-counter abstraction

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>

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 4303b880ac995b051cc0eb632e6b6b58704f9ac2..c62df93bfc1b316a03fe9c6772e83c1c8a9f6550 100644 (file)
--- 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 027e123d93c728af62690ddc762927ab5c33514c..e513b4ab119769488419eba9bff7601f1d556182 100644 (file)
--- 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. */