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Subject: Re: [PATCH v4 1/5] cgroup: subtree_control bypass mode for bypassable
 controllers
To:     Dan Carpenter <dan.carpenter@oracle.com>, kbuild@01.org
Cc:     kbuild-all@01.org, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        Roman Gushchin <guro@fb.com>, Jens Axboe <axboe@kernel.dk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dennis Zhou <dennis@kernel.org>,
        Shakeel Butt <shakeelb@google.com>
References: <20181129111811.GP3073@unbuntlaptop>
From:   Waiman Long <longman@redhat.com>
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Organization: Red Hat
Message-ID: <181c8ea6-7b5a-1ee4-7049-84aad192697c@redhat.com>
Date:   Mon, 10 Dec 2018 14:56:25 -0500
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On 11/29/2018 06:18 AM, Dan Carpenter wrote:
> Hi Waiman,
>
> Thank you for the patch! Perhaps something to improve:
>
> url:    https://github.com/0day-ci/linux/commits/Waiman-Long/cgroup-Introducing-bypass-mode/20181123-030552
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-next
>
> smatch warnings:
> kernel/cgroup/cgroup.c:4893 css_create() error: we previously assumed 'parent' could be null (see line 4864)
>
> # https://github.com/0day-ci/linux/commit/8b68fd4330e043645667a5d3306398f8f88f9ff2
> git remote add linux-review https://github.com/0day-ci/linux
> git remote update linux-review
> git checkout 8b68fd4330e043645667a5d3306398f8f88f9ff2
> vim +/parent +4893 kernel/cgroup/cgroup.c
>
> a31f2d3ff kernel/cgroup.c        Tejun Heo        2012-11-19  4840  
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4841  /**
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4842   * css_create - create a cgroup_subsys_state
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4843   * @cgrp: the cgroup new css will be associated with
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4844   * @ss: the subsys of new css
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4845   *
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4846   * Create a new css associated with @cgrp - @ss pair.  On success, the new
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4847   * css is online and installed in @cgrp.  This function doesn't create the
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4848   * interface files.  Returns 0 on success, -errno on failure.
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4849   */
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4850  static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4851  					      struct cgroup_subsys *ss)
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4852  {
> d51f39b05 kernel/cgroup.c        Tejun Heo        2014-05-16  4853  	struct cgroup *parent = cgroup_parent(cgrp);
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4854  	struct cgroup_subsys_state *parent_css = NULL;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4855  	struct cgroup_subsys_state *css;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4856  	int err;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4857  
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4858  	lockdep_assert_held(&cgroup_mutex);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4859  
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4860  	/*
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4861  	 * As cgroup may be in bypass mode, need to skip over ancestor
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4862  	 * cgroups with NULL CSS.
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4863  	 */
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20 @4864  	for (; parent && !parent_css; parent = cgroup_parent(parent))
>                                                                                ^^^^^^^^^^^^^^^^^^^^^
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4865  		parent_css = cgroup_css(parent, ss);
>
> When we exit this loop it means either parent is NULL or parent_css
> is non-NULL.

Sorry for the bug. I should have a temp variable to go up
cgroup_parent() iteration chain and leave parent alone. Thanks for
spotting that.

> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4866  
> 1fed1b2e3 kernel/cgroup.c        Tejun Heo        2014-05-16  4867  	css = ss->css_alloc(parent_css);
> e7e15b87f kernel/cgroup.c        Tejun Heo        2016-06-21  4868  	if (!css)
> e7e15b87f kernel/cgroup.c        Tejun Heo        2016-06-21  4869  		css = ERR_PTR(-ENOMEM);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4870  	if (IS_ERR(css))
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4871  		return css;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4872  
> 8b68fd433 kernel/cgroup/cgroup.c Waiman Long      2018-11-20  4873  	init_and_link_css(css, ss, cgrp, parent_css);
> a2bed8209 kernel/cgroup.c        Tejun Heo        2014-05-04  4874  
> 2aad2a86f kernel/cgroup.c        Tejun Heo        2014-09-24  4875  	err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4876  	if (err)
> 3eb59ec64 kernel/cgroup.c        Li Zefan         2014-03-18  4877  		goto err_free_css;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4878  
> cf780b7dc kernel/cgroup.c        Vladimir Davydov 2015-08-03  4879  	err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
> 15a4c835e kernel/cgroup.c        Tejun Heo        2014-05-04  4880  	if (err < 0)
> b00c52dae kernel/cgroup.c        Wenwei Tao       2016-05-13  4881  		goto err_free_css;
> 15a4c835e kernel/cgroup.c        Tejun Heo        2014-05-04  4882  	css->id = err;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4883  
> 15a4c835e kernel/cgroup.c        Tejun Heo        2014-05-04  4884  	/* @css is ready to be brought online now, make it visible */
> 1fed1b2e3 kernel/cgroup.c        Tejun Heo        2014-05-16  4885  	list_add_tail_rcu(&css->sibling, &parent_css->children);
> 15a4c835e kernel/cgroup.c        Tejun Heo        2014-05-04  4886  	cgroup_idr_replace(&ss->css_idr, css, css->id);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4887  
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4888  	err = online_css(css);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4889  	if (err)
> 1fed1b2e3 kernel/cgroup.c        Tejun Heo        2014-05-16  4890  		goto err_list_del;
> 944196278 kernel/cgroup.c        Tejun Heo        2014-03-19  4891  
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4892  	if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
> d51f39b05 kernel/cgroup.c        Tejun Heo        2014-05-16 @4893  	    cgroup_parent(parent)) {
>                                                                                           ^^^^^^
> We dereference parent inside the function, but I don't know for sure
> if this is reachable when "parent" is NULL.
>
> ed3d261b5 kernel/cgroup.c        Joe Perches      2014-04-25  4894  		pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4895  			current->comm, current->pid, ss->name);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4896  		if (!strcmp(ss->name, "memory"))
> ed3d261b5 kernel/cgroup.c        Joe Perches      2014-04-25  4897  			pr_warn("\"memory\" requires setting use_hierarchy to 1 on the root\n");
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4898  		ss->warned_broken_hierarchy = true;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4899  	}
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4900  
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4901  	return css;
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4902  
> 1fed1b2e3 kernel/cgroup.c        Tejun Heo        2014-05-16  4903  err_list_del:
> 1fed1b2e3 kernel/cgroup.c        Tejun Heo        2014-05-16  4904  	list_del_rcu(&css->sibling);
> 3eb59ec64 kernel/cgroup.c        Li Zefan         2014-03-18  4905  err_free_css:
> 8f53470ba kernel/cgroup/cgroup.c Tejun Heo        2018-04-26  4906  	list_del_rcu(&css->rstat_css_node);
> 8f36aaec9 kernel/cgroup/cgroup.c Tejun Heo        2018-03-14  4907  	INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
> 8f36aaec9 kernel/cgroup/cgroup.c Tejun Heo        2018-03-14  4908  	queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
> 6cd0f5bba kernel/cgroup.c        Tejun Heo        2016-03-03  4909  	return ERR_PTR(err);
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4910  }
> c81c925ad kernel/cgroup.c        Tejun Heo        2013-12-06  4911  
>
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Cheers,
Longman