linux-kernel - [PATCH tip/core/rcu 18/18] rcu: Correct READ_ONCE()/WRITE_ONCE() for ->rcu_read_unlock

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Message-Id: <20190326231314.19518-18-paulmck@linux.ibm.com>
Date:   Tue, 26 Mar 2019 16:13:14 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 18/18] rcu: Correct READ_ONCE()/WRITE_ONCE() for ->rcu_read_unlock_special

The task_struct structure's ->rcu_read_unlock_special field is only ever
read or written by the owning task, but it is accessed both at process
and interrupt levels.  It may therefore be accessed using plain reads
and writes while interrupts are disabled, but must be accessed using
READ_ONCE() and WRITE_ONCE() or better otherwise.  This commit makes a
few adjustments to align with this discipline.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_exp.h    | 2 +-
 kernel/rcu/tree_plugin.h | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index ec4fb93a5dbe..1ee0782213b8 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -633,7 +633,7 @@ static void rcu_exp_handler(void *unused)
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 		if (rnp->expmask & rdp->grpmask) {
 			rdp->deferred_qs = true;
-			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
+			t->rcu_read_unlock_special.b.exp_hint = true;
 		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		return;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 81d3cd821891..6ddb3c05e88f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -285,7 +285,7 @@ static void rcu_qs(void)
 				       TPS("cpuqs"));
 		__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
 		barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
-		current->rcu_read_unlock_special.b.need_qs = false;
+		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
 	}
 }
 
@@ -817,7 +817,7 @@ void exit_rcu(void)
 	if (unlikely(!list_empty(&current->rcu_node_entry))) {
 		t->rcu_read_lock_nesting = 1;
 		barrier();
-		t->rcu_read_unlock_special.b.blocked = true;
+		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
 	} else if (unlikely(t->rcu_read_lock_nesting)) {
 		t->rcu_read_lock_nesting = 1;
 	} else {
-- 
2.17.1