[patch 3/3] generic-ipi: fix the smp_mb() usage

[Suresh Siddha <suresh.b.siddha@intel.com>]

[-- Attachment #1: generic_ipi_weaken_barrier.patch --]
[-- Type: text/plain, Size: 1331 bytes --]

Remove the unnecessary smp_mb() in csd_flag_wait(), as the presence of that
smp_mb() doesn't mean anything on the sender, when the ipi receiver is not
doing any thing special (like memory fence) after clearing the CSD_FLAG_WAIT.

IPI receiver (on Alpha atleast) needs smp_read_barrier_depends() in the handler
before reading the call_single_queue list in a lock-free fashion. Replace
the existing smp_mb() (which is unnecessary) with smp_read_barrier_depends()

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

Index: linux-2.6.git/kernel/smp.c
===================================================================
--- linux-2.6.git.orig/kernel/smp.c	2008-11-04 09:54:29.000000000 -0800
+++ linux-2.6.git/kernel/smp.c	2008-11-04 10:04:54.000000000 -0800
@@ -51,10 +51,6 @@
 {
 	/* Wait for response */
 	do {
-		/*
-		 * We need to see the flags store in the IPI handler
-		 */
-		smp_mb();
 		if (!(data->flags & CSD_FLAG_WAIT))
 			break;
 		cpu_relax();
@@ -162,7 +158,7 @@
 	 * Need to see other stores to list head for checking whether
 	 * list is empty without holding q->lock
 	 */
-	smp_mb();
+	smp_read_barrier_depends();
 	while (!list_empty(&q->list)) {
 		unsigned int data_flags;
 
@@ -196,7 +192,7 @@
 		/*
 		 * See comment on outer loop
 		 */
-		smp_mb();
+		smp_read_barrier_depends();
 	}
 }
 

--