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    SubjectRe: [PATCH] perf: Fix RCU dereference check in perf_event_comm
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    On Thu, 2012-03-22 at 13:36 +0200, Ari Savolainen wrote:
    > 22. maaliskuuta 2012 11.53 Peter Zijlstra <a.p.zijlstra@chello.nl> kirjoitti:
    > > On Thu, 2012-03-22 at 01:43 +0200, Ari Savolainen wrote:
    > >> The warning below is printed when executing a command like
    > >> sudo perf record su - user -c "echo hello"
    > >>
    > >> It's fixed by moving the call of perf_event_comm to be protected
    > >> by the task lock.
    > >
    > > That seems like a rather poor solution since it increases the lock hold
    > > time for no explained reason.
    > >
    > >> include/linux/cgroup.h:567 suspicious rcu_dereference_check() usage!
    > >
    > >> [<ffffffff8109be55>] lockdep_rcu_suspicious+0xe5/0x100
    > >> [<ffffffff811131fa>] perf_event_comm+0x37a/0x4d0
    > >
    > > So where exactly is this, perf_event_comm_event() takes rcu_read_lock()
    > > so I presume its before that.
    >
    > I think the warning comes from this source-level call path:
    >
    > perf_event_comm ->
    > perf_event_enable_on_exec ->
    > perf_cgroup_sched_out ->
    > perf_cgroup_from_task ->
    > task_subsys_state ->
    > task_subsys_state_check
    >
    > It seems there that path does not take rcu_read_lock(). Where should
    > rcu_read_lock/unlock be added? In perf_group_sched_out around the
    > calls of perf_cgroup_from_task? Like this:

    Ah, ok. So IIRC this too is not needed. As the comment near
    perf_cgroup_from_task() says, we hold explicit references to the cgroup.

    Ideally we'd come up with a better validation condition but all variants
    I could come up with make the code ugly and might actually generate
    worse code, the current true simply shuts it up.

    Stephane any thoughts?

    ---
    kernel/events/core.c | 2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)

    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index a6a9ec4..e423261 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -240,7 +240,7 @@ static void perf_ctx_unlock(struct perf_cpu_context *cpuctx,
    static inline struct perf_cgroup *
    perf_cgroup_from_task(struct task_struct *task)
    {
    - return container_of(task_subsys_state(task, perf_subsys_id),
    + return container_of(task_subsys_state_check(task, perf_subsys_id, true),
    struct perf_cgroup, css);
    }



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