Re: [PATCH] cpuset: current_cpuset_is_being_rebound() need rcu lock

[Paul E. McKenney]

On Thu, Mar 11, 2010 at 02:46:15PM +0900, KOSAKI Motohiro wrote:
> rcu lockdep detected cpuset have wrong rcu usage.
> the fixing is trivial. but I wonder why don't cpuset_being_rebound assignment
> and read need a memory barrier pairing?

The fix is in -tip, commit 99ee4ca746dda71326db7645463b4075ac1d665c.

This is an initialization-time use of rcu_dereference(), so no other
task has a reference to this data.  Hence it is constant.  Other uses
of this code operate on shared data structures, which might change at
any time.

							Thanx, Paul

> =============== CUT HERE ==========================================
> Subject: [PATCH] cpuset: current_cpuset_is_being_rebound() need rcu lock.
> 
> Currently, rcu-lockdep display following warning.
> because current_cpuset_is_being_rebound() call task_cs(), but it isn't
> protected by rcu lock.
> 
> This patch fixes it.
> 
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> include/linux/cgroup.h:534 invoked rcu_dereference_check() without
> protection!
> 
> other info that might help us debug this:
> 
> no locks held by swapper/0.
> 
> stack backtrace:
> Pid: 0, comm: swapper Not tainted 2.6.34-rc1-mm1+ #94
> Call Trace:
>  [<ffffffff81086961>] lockdep_rcu_dereference+0xa1/0xb0
>  [<ffffffff810a8cba>] current_cpuset_is_being_rebound+0x7a/0x80
>  [<ffffffff8112ae0a>] __mpol_dup+0x3a/0xa0
>  [<ffffffff8143b9e9>] ? sub_preempt_count+0x9/0xa0
>  [<ffffffff81438105>] ? _raw_spin_unlock+0x35/0x60
>  [<ffffffff810a29dd>] ? cgroup_fork+0x4d/0x70
>  [<ffffffff8104d1b0>] copy_process+0x530/0x1360
>  [<ffffffff8104e067>] do_fork+0x87/0x470
>  [<ffffffff8100a8a7>] ? native_sched_clock+0x27/0x80
>  [<ffffffff81078adf>] ? cpu_clock+0x4f/0x60
>  [<ffffffff81435b2e>] ? mutex_unlock+0xe/0x10
>  [<ffffffff81084c09>] ? trace_hardirqs_off_caller+0x29/0xe0
>  [<ffffffff8143b9e9>] ? sub_preempt_count+0x9/0xa0
>  [<ffffffff8108579e>] ? put_lock_stats+0xe/0x30
>  [<ffffffff8100b5c1>] kernel_thread+0x71/0x80
>  [<ffffffff81b465b3>] ? kernel_init+0x0/0x253
>  [<ffffffff81003f10>] ? kernel_thread_helper+0x0/0x10
>  [<ffffffff8106ea24>] ? rcu_scheduler_starting+0x24/0x60
>  [<ffffffff8141c6a6>] rest_init+0x26/0x110
>  [<ffffffff81b46dea>] start_kernel+0x3b9/0x3c5
>  [<ffffffff81b46310>] x86_64_start_reservations+0x120/0x124
>  [<ffffffff81b463f8>] x86_64_start_kernel+0xe4/0xeb
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Cc: Miao Xie <miaox@xxxxxxxxxxxxxx>
> Cc: Paul Menage <menage@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxx>
> ---
>  kernel/cpuset.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index b15c01c..4d44f76 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -1129,7 +1129,14 @@ done:
> 
>  int current_cpuset_is_being_rebound(void)
>  {
> -	return task_cs(current) == cpuset_being_rebound;
> +	int being_rebound = 0;
> +
> +	rcu_read_lock();
> +	if (task_cs(current) == cpuset_being_rebound)
> +		being_rebound = 1;
> +	rcu_read_unlock();
> +
> +	return being_rebound;
>  }
> 
>  static int update_relax_domain_level(struct cpuset *cs, s64 val)
> -- 
> 1.6.5.2
> 
> 
> 
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