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    SubjectRe: INFO: suspicious rcu_dereference_check() usage - include/linux/cgroup.h:492 invoked rcu_dereference_check() without protection!
    On Mon, Apr 19, 2010 at 02:26:52PM -0400, Eric Paris wrote:
    > On Wed, 2010-04-14 at 12:47 +0200, Peter Zijlstra wrote:
    > > On Mon, 2010-04-12 at 20:47 +0200, Peter Zijlstra wrote:
    > > > On Mon, 2010-04-12 at 14:44 -0400, Eric Paris wrote:
    > > > > On Wed, Mar 10, 2010 at 11:28 PM, Paul E. McKenney
    > > > > <paulmck@linux.vnet.ibm.com> wrote:
    > > >
    > > > > I know you indicated this was fixed in mainline and I see that set of
    > > > > commits objects, but I'm seeing the below spew from linux-next today.
    > > > >
    > > > > tree: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
    > > > > tag: next-20100412
    > > > > commit: bbeecf185fe464ccd7ee97ce6d3646ad686995b4
    > > > >
    > > > > [ 0.035602] ===================================================
    > > > > [ 0.036003] [ INFO: suspicious rcu_dereference_check() usage. ]
    > > > > [ 0.037006] ---------------------------------------------------
    > > > > [ 0.038004] include/linux/cgroup.h:533 invoked
    > > > > rcu_dereference_check() without protection!
    > > > > [ 0.039003]
    > > > > [ 0.039004] other info that might help us debug this:
    > > > > [ 0.039004]
    > > > > [ 0.040003]
    > > > > [ 0.040004] rcu_scheduler_active = 1, debug_locks = 0
    > > > > [ 0.041004] no locks held by swapper/0.
    > > > > [ 0.042003]
    > > > > [ 0.042004] stack backtrace:
    > > > > [ 0.043005] Pid: 0, comm: swapper Not tainted 2.6.34-rc3-next-20100412+ #65
    > > > > [ 0.044003] Call Trace:
    > > > > [ 0.045015] [<ffffffff8108584f>] lockdep_rcu_dereference+0xaf/0xc0
    > > > > [ 0.046010] [<ffffffff81044812>] set_task_cpu+0x2d2/0x370
    > > >
    > > > Oh, right, I still have to sort that out.
    > > >
    > > > I need to figure out how all that scheduler and cgroup muck interact to
    > > > fix this.
    > >
    > > I think the below should cure this..
    > >
    > >
    > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > > ---
    > > kernel/sched.c | 10 ++++++++++
    > > 1 files changed, 10 insertions(+), 0 deletions(-)
    > >
    > > diff --git a/kernel/sched.c b/kernel/sched.c
    > > index 3acf694..2e06d87 100644
    > > --- a/kernel/sched.c
    > > +++ b/kernel/sched.c
    > > @@ -323,6 +323,15 @@ static inline struct task_group *task_group(struct task_struct *p)
    > > /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
    > > static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
    > > {
    > > + /*
    > > + * Strictly speaking this rcu_read_lock() is not needed since the
    > > + * task_group is tied to the cgroup, which in turn can never go away
    > > + * as long as there are tasks attached to it.
    > > + *
    > > + * However since task_group() uses task_subsys_state() which is an
    > > + * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
    > > + */
    > > + rcu_read_lock();
    > > #ifdef CONFIG_FAIR_GROUP_SCHED
    > > p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
    > > p->se.parent = task_group(p)->se[cpu];
    > > @@ -332,6 +341,7 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
    > > p->rt.rt_rq = task_group(p)->rt_rq[cpu];
    > > p->rt.parent = task_group(p)->rt_se[cpu];
    > > #endif
    > > + rcu_read_unlock();
    > > }
    > >
    > > #else
    >
    > So I'm back with another one even with this patch. Would people prefer
    > another thread?
    >
    > [ 0.037175] ===================================================
    > [ 0.038003] [ INFO: suspicious rcu_dereference_check() usage. ]
    > [ 0.039003] ---------------------------------------------------
    > [ 0.040004] include/linux/cgroup.h:533 invoked rcu_dereference_check() without protection!
    > [ 0.041003]
    > [ 0.041004] other info that might help us debug this:
    > [ 0.041005]
    > [ 0.042004]
    > [ 0.042004] rcu_scheduler_active = 1, debug_locks = 0
    > [ 0.043004] no locks held by swapper/0.
    > [ 0.044003]
    > [ 0.044004] stack backtrace:
    > [ 0.045005] Pid: 0, comm: swapper Not tainted 2.6.34-rc4-next-20100415+ #94
    > [ 0.046004] Call Trace:
    > [ 0.047014] [<ffffffff8108652f>] lockdep_rcu_dereference+0xaf/0xc0
    > [ 0.048013] [<ffffffff810a3453>] freezer_fork+0xb3/0xd0
    > [ 0.049007] [<ffffffff8109d61c>] cgroup_fork_callbacks+0x2c/0x40
    > [ 0.050007] [<ffffffff81055e4a>] copy_process+0xb6a/0x11e0
    > [ 0.051006] [<ffffffff8105657e>] do_fork+0xbe/0x3e0
    > [ 0.052007] [<ffffffff81012519>] ? sched_clock+0x9/0x10
    > [ 0.053008] [<ffffffff81077d45>] ? sched_clock_local+0x15/0x80
    > [ 0.054006] [<ffffffff81077e69>] ? sched_clock_cpu+0xb9/0xf0
    > [ 0.055006] [<ffffffff81076cd5>] ? up+0x35/0x50
    > [ 0.056006] [<ffffffff81084073>] ? get_lock_stats+0x23/0x70
    > [ 0.057006] [<ffffffff810840ce>] ? put_lock_stats+0xe/0x30
    > [ 0.058010] [<ffffffff81cade20>] ? kernel_init+0x0/0x2e0
    > [ 0.059006] [<ffffffff810136dd>] kernel_thread+0x8d/0xa0
    > [ 0.060006] [<ffffffff81cade20>] ? kernel_init+0x0/0x2e0
    > [ 0.061007] [<ffffffff8100bc20>] ? kernel_thread_helper+0x0/0x10
    > [ 0.062006] [<ffffffff81cad140>] ? early_idt_handler+0x0/0x71
    > [ 0.063011] [<ffffffff814e40c1>] rest_init+0x21/0x110
    > [ 0.064005] [<ffffffff81cadd3f>] start_kernel+0x3af/0x490
    > [ 0.065006] [<ffffffff81cad29c>] x86_64_start_reservations+0x7c/0xd0
    > [ 0.066005] [<ffffffff81cad000>] ? early_idt_handlers+0x0/0x140
    > [ 0.067006] [<ffffffff81cad3e8>] x86_64_start_kernel+0xf8/0x130

    Yep, different code path to the same location. Does the following
    patch help?

    Thanx, Paul

    ------------------------------------------------------------------------

    commit 2836f18139267ea918ed2cf39023fb0eb38c4361
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Date: Mon Apr 19 15:59:50 2010 -0700

    rcu: fix RCU lockdep splat on freezer_fork path

    Add an RCU read-side critical section to suppress this false positive.

    Located-by: Eric Paris <eparis@parisplace.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
    index da5e139..e5c0244 100644
    --- a/kernel/cgroup_freezer.c
    +++ b/kernel/cgroup_freezer.c
    @@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
    * No lock is needed, since the task isn't on tasklist yet,
    * so it can't be moved to another cgroup, which means the
    * freezer won't be removed and will be valid during this
    - * function call.
    + * function call. Nevertheless, apply RCU read-side critical
    + * section to suppress RCU lockdep false positives.
    */
    + rcu_read_lock();
    freezer = task_freezer(task);
    + rcu_read_unlock();

    /*
    * The root cgroup is non-freezable, so we can skip the

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