linux-kernel - [PATCH tip/core/rcu 01/17] rcu: Avoid ->dynticks_nmi

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Date:   Fri,  1 Dec 2017 11:36:32 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     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,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 01/17] rcu: Avoid ->dynticks_nmi_nesting store tearing

NMIs can nest, and store tearing could in theory happen on carries
from one byte to the next.  This commit therefore adds the WRITE_ONCE()
macros preventing this.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f9c0ca2ccf0c..c5d960f86cf8 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1103,7 +1103,8 @@ void rcu_nmi_enter(void)
 		rcu_dynticks_eqs_exit();
 		incby = 1;
 	}
-	rdtp->dynticks_nmi_nesting += incby;
+	WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* Prevent store tearing. */
+		   rdtp->dynticks_nmi_nesting + incby);
 	barrier();
 }
 
@@ -1135,12 +1136,13 @@ void rcu_nmi_exit(void)
 	 * leave it in non-RCU-idle state.
 	 */
 	if (rdtp->dynticks_nmi_nesting != 1) {
-		rdtp->dynticks_nmi_nesting -= 2;
+		WRITE_ONCE(rdtp->dynticks_nmi_nesting, /* No store tearing. */
+			   rdtp->dynticks_nmi_nesting - 2);
 		return;
 	}
 
 	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
-	rdtp->dynticks_nmi_nesting = 0;
+	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
 	rcu_dynticks_eqs_enter();
 }
 
-- 
2.5.2