rcu: Avoid ->dynticks_nesting store tearing

Although ->dynticks_nesting is updated only by process level, it is
accessed from hardirq to check for interrupt-from-idle quiescent states.
Store tearing is thus possible, so this commit applies WRITE_ONCE()
to ->dynticks_nesting stores.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 80cada1..b2ded4d 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -778,7 +778,7 @@ static void rcu_eqs_enter_common(bool user)
                do_nocb_deferred_wakeup(rdp);
        }
        rcu_prepare_for_idle();
-       rdtp->dynticks_nesting = 0;
+       WRITE_ONCE(rdtp->dynticks_nesting, 0); /* Avoid irq-access tearing. */
        rcu_dynticks_eqs_enter();
        rcu_dynticks_task_enter();
 }
@@ -976,7 +976,7 @@ static void rcu_eqs_exit(bool user)
                rdtp->dynticks_nesting++;
        } else {
                rcu_eqs_exit_common(1, user);
-               rdtp->dynticks_nesting = 1;
+               WRITE_ONCE(rdtp->dynticks_nesting, 1);
                WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
        }
 }
@@ -3713,7 +3713,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
        if (rcu_segcblist_empty(&rdp->cblist) && /* No early-boot CBs? */
            !init_nocb_callback_list(rdp))
                rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
-       rdp->dynticks->dynticks_nesting = 1;
+       rdp->dynticks->dynticks_nesting = 1;    /* CPU not up, no tearing. */
        rcu_dynticks_eqs_online();
        raw_spin_unlock_rcu_node(rnp);          /* irqs remain disabled. */