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>

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index fe85600ba8c2..78923a50cdb2 100644
--- 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);