perf_events: Fix stale ->cgrp pointer in update_cgrp_time_from_cpuctx()

This patch solves a stale pointer problem in
update_cgrp_time_from_cpuctx(). The cpuctx->cgrp
was not cleared on all possible event exit paths,
including:

   close()
     perf_release()
       perf_release_kernel()
         list_del_event()

This patch fixes list_del_event() to clear cpuctx->cgrp
when there are no cgroup events left in the context.

[ This second version makes the code compile when
  CONFIG_CGROUP_PERF is not enabled. We unconditionally define
  perf_cpu_context->cgrp. ]

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: peterz@infradead.org
Cc: perfmon2-devel@lists.sf.net
Cc: paulus@samba.org
Cc: davem@davemloft.net
LKML-Reference: <20110323150306.GA1580@quad>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 1 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 3472bb1a070c..0c714226ae0c 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -941,6 +941,7 @@ static void perf_group_attach(struct perf_event *event)
 static void
 list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 {
+        struct perf_cpu_context *cpuctx;
         /*
          * We can have double detach due to exit/hot-unplug + close.
          */
@@ -949,8 +950,17 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
 
         event->attach_state &= ~PERF_ATTACH_CONTEXT;
 
-        if (is_cgroup_event(event))
+        if (is_cgroup_event(event)) {
                 ctx->nr_cgroups--;
+                cpuctx = __get_cpu_context(ctx);
+                /*
+                 * if there are no more cgroup events
+                 * then cler cgrp to avoid stale pointer
+                 * in update_cgrp_time_from_cpuctx()
+                 */
+                if (!ctx->nr_cgroups)
+                        cpuctx->cgrp = NULL;
+        }
 
         ctx->nr_events--;
         if (event->attr.inherit_stat)