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Subject[PATCH 1/2] perf: fix find_get_context() vs perf_event_exit_task() race
find_get_context() must not install the new perf_event_context if the
task has already passed perf_event_exit_task().

If nothing else, this means the memory leak. Initially ctx->refcount == 2,
it is supposed that perf_event_exit_task_context() should participate and
do the necessary put_ctx().

find_lively_task_by_vpid() checks PF_EXITING but this buys nothing, by the
time we call find_get_context() this task can be already dead. To the point,
cmpxchg() can succeed when the task has already done the last schedule().

Change find_get_context() to populate task->perf_event_ctxp[] under
task->perf_event_mutex, this way we can trust PF_EXITING because
perf_event_exit_task() takes the same mutex.

Also, change perf_event_exit_task_context() to use rcu_dereference().
Probably this is not strictly needed, but with or without this change
find_get_context() can race with setup_new_exec()->perf_event_exit_task(),
rcu_dereference() looks better.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/perf_event.c | 34 ++++++++++++++++++++--------------
1 file changed, 20 insertions(+), 14 deletions(-)
--- git/kernel/perf_event.c~3_find_get_context_vs_exit 2011-01-18 16:57:08.000000000 +0100
+++ git/kernel/perf_event.c 2011-01-19 17:41:16.000000000 +0100
@@ -2201,13 +2201,6 @@ find_lively_task_by_vpid(pid_t vpid)
if (!task)
return ERR_PTR(-ESRCH);

- /*
- * Can't attach events to a dying task.
- */
- err = -ESRCH;
- if (task->flags & PF_EXITING)
- goto errout;
-
/* Reuse ptrace permission checks for now. */
err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
@@ -2268,14 +2261,27 @@ retry:

get_ctx(ctx);

- if (cmpxchg(&task->perf_event_ctxp[ctxn], NULL, ctx)) {
- /*
- * We raced with some other task; use
- * the context they set.
- */
+ err = 0;
+ mutex_lock(&task->perf_event_mutex);
+ /*
+ * If it has already passed perf_event_exit_task().
+ * we must see PF_EXITING, it takes this mutex too.
+ */
+ if (task->flags & PF_EXITING)
+ err = -ESRCH;
+ else if (task->perf_event_ctxp[ctxn])
+ err = -EAGAIN;
+ else
+ rcu_assign_pointer(task->perf_event_ctxp[ctxn], ctx);
+ mutex_unlock(&task->perf_event_mutex);
+
+ if (unlikely(err)) {
put_task_struct(task);
kfree(ctx);
- goto retry;
+
+ if (err == -EAGAIN)
+ goto retry;
+ goto errout;
}
}

@@ -6127,7 +6133,7 @@ static void perf_event_exit_task_context
* scheduled, so we are now safe from rescheduling changing
* our context.
*/
- child_ctx = child->perf_event_ctxp[ctxn];
+ child_ctx = rcu_dereference(child->perf_event_ctxp[ctxn]);
task_ctx_sched_out(child_ctx, EVENT_ALL);

/*


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