rcu: Remove local_irq_disable() in rcu_preempt_note_context_switch()

The rcu_preempt_note_context_switch() function is on a scheduling fast
path, so it would be good to avoid disabling irqs.  The reason that irqs
are disabled is to synchronize process-level and irq-handler access to
the task_struct ->rcu_read_unlock_special bitmask.  This commit therefore
makes ->rcu_read_unlock_special instead be a union of bools with a short
allowing single-access checks in RCU's __rcu_read_unlock().  This results
in the process-level and irq-handler accesses being simple loads and
stores, so that irqs need no longer be disabled.  This commit therefore
removes the irq disabling from rcu_preempt_note_context_switch().

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

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 9487b4898e51..6fb911558562 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -93,7 +93,7 @@ void __rcu_read_unlock(void)
                 barrier();  /* critical section before exit code. */
                 t->rcu_read_lock_nesting = INT_MIN;
                 barrier();  /* assign before ->rcu_read_unlock_special load */
-                if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
+                if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s)))
                         rcu_read_unlock_special(t);
                 barrier();  /* ->rcu_read_unlock_special load before assign */
                 t->rcu_read_lock_nesting = 0;