[PATCH/RFC 1/4] stop_machine: atomic update for combined return value

[Heiko Carstens <heiko.carstens@de.ibm.com>]

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From: Heiko Carstens <heiko.carstens@de.ibm.com>

Using |= for updating a value which might be updated on several cpus
concurrently will not always work since we need to make sure that the
update happens atomically. So let's use a cmpxchg loop instead.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
 kernel/stop_machine.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6/kernel/stop_machine.c
===================================================================
--- linux-2.6.orig/kernel/stop_machine.c
+++ linux-2.6/kernel/stop_machine.c
@@ -65,6 +65,7 @@ static void ack_state(void)
 static int stop_cpu(struct stop_machine_data *smdata)
 {
 	enum stopmachine_state curstate = STOPMACHINE_NONE;
+	int *frp, fnret, old;
 
 	/* Simple state machine */
 	do {
@@ -80,7 +81,11 @@ static int stop_cpu(struct stop_machine_
 			case STOPMACHINE_RUN:
 				/* |= allows error detection if functions on
 				 * multiple CPUs. */
-				smdata->fnret |= smdata->fn(smdata->data);
+				fnret = smdata->fn(smdata->data);
+				frp = &smdata->fnret;
+				do {
+					old = *frp;
+				} while (cmpxchg(frp, old, old | fnret) != old);
 				break;
 			default:
 				break;

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