markers: fix markers read barrier for multiple probes

commit 5def9a3a22e09c99717f41ab7f07ec9e1a1f3ec8 upstream

Paul pointed out two incorrect read barriers in the marker handler code in
the path where multiple probes are connected.  Those are ordering reads of
"ptype" (single or multi probe marker), "multi" array pointer, and "multi"
array data access.

It should be ordered like this :

read ptype
smp_rmb()
read multi array pointer
smp_read_barrier_depends()
access data referenced by multi array pointer

The code with a single probe connected (optimized case, does not have to
allocate an array) has correct memory ordering.

It applies to kernel 2.6.26.x, 2.6.25.x and linux-next.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/marker.c b/kernel/marker.c
index 005b959545933703bbb76e3f9720c5e42466df99..54ce00f810e1a1443daf852d93d2102ef1ea214e 100644 (file)
--- a/kernel/marker.c
+++ b/kernel/marker.c
@@ -125,6 +125,11 @@ void marker_probe_cb(const struct marker *mdata, void *call_private,
        } else {
                struct marker_probe_closure *multi;
                int i;
+               /*
+                * Read mdata->ptype before mdata->multi.
+                */
+               smp_rmb();
+               multi = mdata->multi;
                /*
                 * multi points to an array, therefore accessing the array
                 * depends on reading multi. However, even in this case,
@@ -133,7 +138,6 @@ void marker_probe_cb(const struct marker *mdata, void *call_private,
                 * in the fast path, so put the explicit barrier here.
                 */
                smp_read_barrier_depends();
-               multi = mdata->multi;
                for (i = 0; multi[i].func; i++) {
                        va_start(args, fmt);
                        multi[i].func(multi[i].probe_private, call_private, fmt,
@@ -175,6 +179,11 @@ void marker_probe_cb_noarg(const struct marker *mdata,
        } else {
                struct marker_probe_closure *multi;
                int i;
+               /*
+                * Read mdata->ptype before mdata->multi.
+                */
+               smp_rmb();
+               multi = mdata->multi;
                /*
                 * multi points to an array, therefore accessing the array
                 * depends on reading multi. However, even in this case,
@@ -183,7 +192,6 @@ void marker_probe_cb_noarg(const struct marker *mdata,
                 * in the fast path, so put the explicit barrier here.
                 */
                smp_read_barrier_depends();
-               multi = mdata->multi;
                for (i = 0; multi[i].func; i++)
                        multi[i].func(multi[i].probe_private, call_private, fmt,
                                &args);