rcu: Pull out wait_event*() condition into helper function

The condition for the wait_event_interruptible_timeout() that waits
to do the next force-quiescent-state scan is a bit ornate:

	((gf = READ_ONCE(rsp->gp_flags)) &
	 RCU_GP_FLAG_FQS) ||
	(!READ_ONCE(rnp->qsmask) &&
	 !rcu_preempt_blocked_readers_cgp(rnp))

This commit therefore pulls this condition out into a helper function
and comments its component conditions.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4b6594c7db5810ac73d0094ea02aed59a19736d4..b2803730ac13c2eb4b46e4003271102528593fc3 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1903,6 +1903,26 @@ static int rcu_gp_init(struct rcu_state *rsp)
        return 1;
 }
 
+/*
+ * Helper function for wait_event_interruptible_timeout() wakeup
+ * at force-quiescent-state time.
+ */
+static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
+{
+       struct rcu_node *rnp = rcu_get_root(rsp);
+
+       /* Someone like call_rcu() requested a force-quiescent-state scan. */
+       *gfp = READ_ONCE(rsp->gp_flags);
+       if (*gfp & RCU_GP_FLAG_FQS)
+               return true;
+
+       /* The current grace period has completed. */
+       if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp))
+               return true;
+
+       return false;
+}
+
 /*
  * Do one round of quiescent-state forcing.
  */
@@ -2067,11 +2087,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                                               TPS("fqswait"));
                        rsp->gp_state = RCU_GP_WAIT_FQS;
                        ret = wait_event_interruptible_timeout(rsp->gp_wq,
-                                       ((gf = READ_ONCE(rsp->gp_flags)) &
-                                        RCU_GP_FLAG_FQS) ||
-                                       (!READ_ONCE(rnp->qsmask) &&
-                                        !rcu_preempt_blocked_readers_cgp(rnp)),
-                                       j);
+                                       rcu_gp_fqs_check_wake(rsp, &gf), j);
                        rsp->gp_state = RCU_GP_DONE_FQS;
                        /* Locking provides needed memory barriers. */
                        /* If grace period done, leave loop. */