Re: [PATCH] sched: fix another race when reading /proc/sched_debug

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sun, 2008-12-14 at 10:54 +0800, Li Zefan wrote:
> > i merged it up in tip/master, could you please check whether it's ok?
> > 
> 
> Sorry, though this patch avoids accessing a half-created cgroup, but I found
> current code may access a cgroup which has been destroyed.
> 
> The simplest fix is to take cgroup_lock() before for_each_leaf_cfs_rq.
> 
> Could you revert this patch and apply the following new one? My box has
> survived for 16 hours with it applied.
> 
> ==========
> 
> From: Li Zefan <lizf@cn.fujitsu.com>
> Date: Sun, 14 Dec 2008 09:53:28 +0800
> Subject: [PATCH] sched: fix another race when reading /proc/sched_debug
> 
> I fixed an oops with the following commit:
> 
> | commit 24eb089950ce44603b30a3145a2c8520e2b55bb1
> | Author: Li Zefan <lizf@cn.fujitsu.com>
> | Date:   Thu Nov 6 12:53:32 2008 -0800
> |
> |    cgroups: fix invalid cgrp->dentry before cgroup has been completely removed
> |
> |    This fixes an oops when reading /proc/sched_debug.
> 
> The above commit fixed a race that reading /proc/sched_debug may access
> NULL cgrp->dentry if a cgroup is being removed (via cgroup_rmdir), but
> hasn't been destroyed (via cgroup_diput).
> 
> But I found there's another different race, in that reading sched_debug
> may access a cgroup which is being created or has been destroyed, and thus
> dereference NULL cgrp->dentry!
> 
> task_group is added to the global list while the cgroup is being created,
> and is removed from the global list while the cgroup is under destruction.
> So running through the list should be protected by cgroup_lock(), if
> cgroup data will be accessed (here by calling cgroup_path).

Can't we detect a dead task-group and skip those instead of adding this
global lock?

> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> ---
>  kernel/sched_fair.c |    6 ++++++
>  kernel/sched_rt.c   |    6 ++++++
>  2 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 98345e4..8b2965b 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1737,9 +1737,15 @@ static void print_cfs_stats(struct seq_file *m, int cpu)
>  {
>  	struct cfs_rq *cfs_rq;
>  
> +	/*
> +	 * Hold cgroup_lock() to avoid calling cgroup_path() with
> +	 * invalid cgroup.
> +	 */
> +	cgroup_lock();
>  	rcu_read_lock();
>  	for_each_leaf_cfs_rq(cpu_rq(cpu), cfs_rq)
>  		print_cfs_rq(m, cpu, cfs_rq);
>  	rcu_read_unlock();
> +	cgroup_unlock();
>  }
>  #endif
> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> index d9ba9d5..e84480d 100644
> --- a/kernel/sched_rt.c
> +++ b/kernel/sched_rt.c
> @@ -1538,9 +1538,15 @@ static void print_rt_stats(struct seq_file *m, int cpu)
>  {
>  	struct rt_rq *rt_rq;
>  
> +	/*
> +	 * Hold cgroup_lock() to avoid calling cgroup_path() with
> +	 * invalid cgroup.
> +	 */
> +	cgroup_lock();
>  	rcu_read_lock();
>  	for_each_leaf_rt_rq(rt_rq, cpu_rq(cpu))
>  		print_rt_rq(m, cpu, rt_rq);
>  	rcu_read_unlock();
> +	cgroup_unlock();
>  }
>  #endif /* CONFIG_SCHED_DEBUG */