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    SubjectRe: [PATCH] perf: Fix RCU dereference check in perf_event_comm
    From
    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:

    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index 1b5c081..bbb4abc 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -409,6 +409,8 @@ static inline void perf_cgroup_sched_out(struct
    task_struct *task,
    struct perf_cgroup *cgrp1;
    struct perf_cgroup *cgrp2 = NULL;

    + rcu_read_lock();
    +
    /*
    * we come here when we know perf_cgroup_events > 0
    */
    @@ -421,6 +423,8 @@ static inline void perf_cgroup_sched_out(struct
    task_struct *task,
    if (next)
    cgrp2 = perf_cgroup_from_task(next);

    + rcu_read_unlock();
    +
    /*
    * only schedule out current cgroup events if we know
    * that we are switching to a different cgroup. Otherwise,
    --

    Ari
    --
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