Messages in this thread Patch in this message | | | Date | Wed, 23 Jul 2008 18:17:14 -0400 | From | Mathieu Desnoyers <> | Subject | [PATCH] Markers : fix markers read barrier for multiple probes |
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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> --- kernel/marker.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
Index: linux-2.6.26-rc7-lttng/kernel/marker.c =================================================================== --- linux-2.6.26-rc7-lttng.orig/kernel/marker.c 2008-06-25 18:09:49.000000000 -0400 +++ linux-2.6.26-rc7-lttng/kernel/marker.c 2008-06-25 18:10:57.000000000 -0400 @@ -127,6 +127,11 @@ 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, * we must insure that the pointer is read _before_ the array @@ -134,7 +139,6 @@ * 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, @@ -177,6 +181,11 @@ 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, * we must insure that the pointer is read _before_ the array @@ -184,7 +193,6 @@ * 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); -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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