rcu: Adjust FQS offline checks for exact online-CPU detection

Commit 7ec99de36f40 ("rcu: Provide exact CPU-online tracking for RCU"),
as its title suggests, got rid of RCU's remaining CPU-hotplug timing
guesswork.  This commit therefore removes the one-jiffy kludge that was
used to paper over this guesswork.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d8245cb..6e4b1dc 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1276,21 +1276,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
                return 1;
        }
 
-       /*
-        * Check for the CPU being offline, but only if the grace period
-        * is old enough.  We don't need to worry about the CPU changing
-        * state: If we see it offline even once, it has been through a
-        * quiescent state.
-        *
-        * The reason for insisting that the grace period be at least
-        * one jiffy old is that CPUs that are not quite online and that
-        * have just gone offline can still execute RCU read-side critical
-        * sections.
-        */
-       if (ULONG_CMP_GE(rdp->rsp->gp_start + 2, jiffies))
-               return 0;  /* Grace period is not old enough. */
-       barrier();
-       if (cpu_is_offline(rdp->cpu)) {
+       /* Check for the CPU being offline. */
+       if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
                trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("ofl"));
                rdp->offline_fqs++;
                return 1;