rcu: prevent call_rcu() from diving into rcu core if irqs disabled

This commit marks a first step towards making call_rcu() have
real-time behavior.  If irqs are disabled, don't dive into the
RCU core.  Later on, this new early exit will wake up the
per-CPU kthread, which first must be modified to handle the
cases involving callback storms.

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

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index fe85600..78923a5 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1839,6 +1839,13 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
        /* Add the callback to our list. */
        *rdp->nxttail[RCU_NEXT_TAIL] = head;
        rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
+       rdp->qlen++;
+
+       /* If interrupts were disabled, don't dive into RCU core. */
+       if (irqs_disabled_flags(flags)) {
+               local_irq_restore(flags);
+               return;
+       }
 
        /*
         * Force the grace period if too many callbacks or too long waiting.
@@ -1847,7 +1854,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
         * invoking force_quiescent_state() if the newly enqueued callback
         * is the only one waiting for a grace period to complete.
         */
-       if (unlikely(++rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {
+       if (unlikely(rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {
 
                /* Are we ignoring a completed grace period? */
                rcu_process_gp_end(rsp, rdp);