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Subject[PATCH] Markers : fix check format with rcu callback race
The fix "markers: fix unchecked format" introduced an RCU callback race. This
patch takes care of calling any pending RCU callback before set_format is
called.


* Lai Jiangshan (laijs@cn.fujitsu.com) wrote:
>
> bit-field is not thread-safe nor smp-safe.
>
> struct marker_entry.rcu_pending is not protected by any lock
> in rcu-callback free_old_closure().
> so we must turn it into a safe type.
>

All struct marker_entry.rcu_pending accesses are done with the
markers_mutex held, except the one done in free_old_closure(). Normally,
there should be a
if (entry->rcu_pending)
rcu_barrier_sched();

At the beginning of each markers_mutex section (just after get_marker())
to make sure any pending callback is executed at that point before any
of rcu_pending or ptype are touched.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Ingo Molnar <mingo@elte.hu>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
---
kernel/marker.c | 24 +++++++++++++-----------
1 file changed, 13 insertions(+), 11 deletions(-)

Index: linux-2.6-lttng/kernel/marker.c
===================================================================
--- linux-2.6-lttng.orig/kernel/marker.c 2008-10-10 01:35:32.000000000 -0400
+++ linux-2.6-lttng/kernel/marker.c 2008-10-10 01:35:32.000000000 -0400
@@ -657,21 +657,23 @@ int marker_probe_register(const char *na
entry = add_marker(name, format);
if (IS_ERR(entry))
ret = PTR_ERR(entry);
- } else if (format) {
- if (!entry->format)
- ret = marker_set_format(&entry, format);
- else if (strcmp(entry->format, format))
- ret = -EPERM;
+ } else {
+ /*
+ * If we detect that a call_rcu is pending for this marker,
+ * make sure it's executed now.
+ */
+ if (entry->rcu_pending)
+ rcu_barrier_sched();
+ if (format) {
+ if (!entry->format)
+ ret = marker_set_format(&entry, format);
+ else if (strcmp(entry->format, format))
+ ret = -EPERM;
+ }
}
if (ret)
goto end;

- /*
- * If we detect that a call_rcu is pending for this marker,
- * make sure it's executed now.
- */
- if (entry->rcu_pending)
- rcu_barrier_sched();
old = marker_entry_add_probe(entry, probe, probe_private);
if (IS_ERR(old)) {
ret = PTR_ERR(old);
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68

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