linux-kernel - Re: [Suggestion] kernel/cgroup.c: about kfree after 'get_new

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Date:	Tue, 7 May 2013 19:01:59 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Chen Gang <gang.chen@...anux.com>
CC:	<tj@...nel.org>, <containers@...ts.linux-foundation.org>,
	<cgroups@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Suggestion] kernel/cgroup.c: about kfree after 'get_new_cssid'

On 2013/5/7 18:46, Chen Gang wrote:
> Hello Maintainers:
> 
> After call get_new_cssid(), I can not find the related free function
> (it seems free_css_id() is for that, but not used).
> 
> The memory location is:
>   get_new_cssid() --> kzalloc() for 'struct css_id'
>   get_new_cssid() --> idr_alloc() for 'ss->idr'
> 
> One work flow:
>   cgroup_load_subsys() --> cgroup_init_idr() --> get_new_cssid()
>   when get_new_cssid() fails, it will:
>   cgroup_load_subsys() --> cgroup_unload_subsys() --> idr_destroy(),
>   and also:
>   cgroup_load_subsys() --> cgroup_unload_subsys() --> ss->css_free();
>     ('css_free' may 'debug_css_free', or 'freezer_css_free' ...)
> 
> It seems the work flow above is not 'kfree' 'struct css_id', is it true?
> 
> BTW: I also guess, for cgroup_init_idr() in cgroup_init(), need check
> the return value.
> 
> Please help check.
> 

It's the specific cgroup subsystem that calls free_css_id() in it's subsys->css_free()
callback. See __mem_cgroup_free() for example.

There's a bug in cgroup_unload_subsys() that idr_destroy() should be called after
ss->css_free(). That said, given there's no modular cgroup subsystem using css_id,
and the whole css_id thing will be eliminated in 3.11, why bother fixing it.

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