xenbus: Clarify and simplify barrier usage.

Signed-off-by: Keir Fraser <keir@xensource.com>

diff --git a/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_comms.c b/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_comms.c
index c00aaf7f6f5b39c7476f31e4a99e3b4f3d63b70b..2521b8203de2e09b15f909ed76a7995960636a23 100644 (file)
--- a/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_comms.c
+++ b/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_comms.c
@@ -110,7 +110,6 @@ int xb_write(const void *data, unsigned len)
                /* Read indexes, then verify. */
                cons = intf->req_cons;
                prod = intf->req_prod;
-               mb();
                if (!check_indexes(cons, prod)) {
                        intf->req_cons = intf->req_prod = 0;
                        return -EIO;
@@ -122,15 +121,18 @@ int xb_write(const void *data, unsigned len)
                if (avail > len)
                        avail = len;
 
+               /* Must write data /after/ reading the consumer index. */
+               mb();
+
                memcpy(dst, data, avail);
                data += avail;
                len -= avail;
 
-               /* Other side must not see new header until data is there. */
+               /* Other side must not see new producer until data is there. */
                wmb();
                intf->req_prod += avail;
 
-               /* This implies mb() before other side sees interrupt. */
+               /* Implies mb(): other side will see the updated producer. */
                notify_remote_via_evtchn(xen_store_evtchn);
        }
 
@@ -165,7 +167,6 @@ int xb_read(void *data, unsigned len)
                /* Read indexes, then verify. */
                cons = intf->rsp_cons;
                prod = intf->rsp_prod;
-               mb();
                if (!check_indexes(cons, prod)) {
                        intf->rsp_cons = intf->rsp_prod = 0;
                        return -EIO;
@@ -177,7 +178,7 @@ int xb_read(void *data, unsigned len)
                if (avail > len)
                        avail = len;
 
-               /* We must read header before we read data. */
+               /* Must read data /after/ reading the producer index. */
                rmb();
 
                memcpy(data, src, avail);
@@ -190,7 +191,7 @@ int xb_read(void *data, unsigned len)
 
                pr_debug("Finished read of %i bytes (%i to go)\n", avail, len);
 
-               /* Implies mb(): they will see new header. */
+               /* Implies mb(): other side will see the updated consumer. */
                notify_remote_via_evtchn(xen_store_evtchn);
        }