Re: [RFC][PATCH] sched: Protect call to ss->fork() with css_set_rwsem

[Kirill Tkhai <ktkhai@parallels.com>]

Hi, Steven,

В Пн, 10/11/2014 в 23:39 -0500, Steven Rostedt пишет:
> My tests tripped over the following RCU lockdep splat:
> 
> ===============================
> [ INFO: suspicious RCU usage. ]^M
> 3.18.0-rc4-test+ #3 Not tainted
> -------------------------------
> /work/autotest/nobackup/linux-test.git/kernel/sched/core.c:7449 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 0
> 2 locks held by swapper/0/0:
>  #0:  (&p->pi_lock){......}, at: [<ffffffff8105c470>] task_rq_lock+0x33/0x9b
>  #1:  (&rq->lock){-.-...}, at: [<ffffffff8105c48a>] task_rq_lock+0x4d/0x9b
> 
> stack backtrace:
> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.18.0-rc4-test+ #3
> Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
>  0000000000000001 ffffffff82203cd8 ffffffff81a2bb53 0000000000000de1
>  ffffffff822164a0 ffffffff82203d08 ffffffff81075ada ffff880215118000
>  ffff88021ea13a00 0000000000000000 0000000000000000 ffffffff82203d58
> Call Trace:
>  [<ffffffff81a2bb53>] dump_stack+0x46/0x58
>  [<ffffffff81075ada>] lockdep_rcu_suspicious+0x107/0x110
>  [<ffffffff81064a0e>] sched_move_task+0xdd/0x153
>  [<ffffffff81064aab>] cpu_cgroup_fork+0xe/0x10
>  [<ffffffff810b3e8b>] cgroup_post_fork+0x83/0xa5
>  [<ffffffff8103d69d>] copy_process+0x169b/0x1870
>  [<ffffffff81a1f440>] ? rest_init+0x134/0x134
>  [<ffffffff81078117>] ? trace_hardirqs_on_caller+0x160/0x197
>  [<ffffffff812f84be>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>  [<ffffffff8103d995>] do_fork+0x7c/0x23b
>  [<ffffffff81a34c63>] ? mutex_unlock+0xe/0x10
>  [<ffffffff810d259c>] ? ftrace_process_locs+0x482/0x4cd
>  [<ffffffff8105bda9>] ? cpumask_next+0x19/0x1b
>  [<ffffffff8103db7a>] kernel_thread+0x26/0x28
>  [<ffffffff81a1f332>] rest_init+0x26/0x134
>  [<ffffffff823e77eb>] ? ftrace_init+0xad/0x140
>  [<ffffffff823c8eab>] start_kernel+0x42c/0x439
>  [<ffffffff823c8887>] ? set_init_arg+0x55/0x55
>  [<ffffffff823c8491>] x86_64_start_reservations+0x2a/0x2c
>  [<ffffffff823c8585>] x86_64_start_kernel+0xf2/0xf9
> 
> I did a bisect and came across this commit:
> 
> Commit eeb61e53ea19 "sched: Fix race between task_group and
> sched_task_group".
> 
> It added a callback of the fork() method which calls sched_move_task().
> But this function does a task_css_check() which expects to have one of
> these locks held: cgroup_mutex, css_set_rwsem, or siglock (for this
> case), or at least have the task exiting.

I've fixed this, the fix is in tip tree:

https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=sched/urgent&id=f7b8a47da17c9ee4998f2ca2018fcc424e953c0e


> By moving the taking of the css_set_rwsem semaphore out to include the
> call to ss->fork(), it quiets this warning.
> 
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
> 
> I'm not sure this is the correct fix, but it "works for me".
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 136eceadeed1..a08ad94d62cf 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -5150,17 +5150,16 @@ void cgroup_post_fork(struct task_struct *child)
>  	 * in the init_css_set before cg_links is enabled and there's no
>  	 * operation which transfers all tasks out of init_css_set.
>  	 */
> +	down_write(&css_set_rwsem);
>  	if (use_task_css_set_links) {
>  		struct css_set *cset;
>  
> -		down_write(&css_set_rwsem);
>  		cset = task_css_set(current);
>  		if (list_empty(&child->cg_list)) {
>  			rcu_assign_pointer(child->cgroups, cset);
>  			list_add(&child->cg_list, &cset->tasks);
>  			get_css_set(cset);
>  		}
> -		up_write(&css_set_rwsem);
>  	}
>  
>  	/*
> @@ -5173,6 +5172,7 @@ void cgroup_post_fork(struct task_struct *child)
>  			if (ss->fork)
>  				ss->fork(child);
>  	}
> +	up_write(&css_set_rwsem);
>  }
>  

>  /**

Kirill