linux-kernel - Re: [PATCHv6 1/3] rdmacg: Added rdma cgroup controller

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Date:	Thu, 25 Feb 2016 16:30:45 +0200
From:	Haggai Eran <haggaie@...lanox.com>
To:	Parav Pandit <pandit.parav@...il.com>
CC:	<cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Doug Ledford <dledford@...hat.com>,
	Liran Liss <liranl@...lanox.com>,
	"Hefty, Sean" <sean.hefty@...el.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Jonathan Corbet <corbet@....net>, <james.l.morris@...cle.com>,
	<serge@...lyn.com>, Or Gerlitz <ogerlitz@...lanox.com>,
	Matan Barak <matanb@...lanox.com>, <raindel@...lanox.com>,
	<akpm@...ux-foundation.org>,
	<linux-security-module@...r.kernel.org>
Subject: Re: [PATCHv6 1/3] rdmacg: Added rdma cgroup controller

On 25/02/2016 15:34, Parav Pandit wrote:
> On Thu, Feb 25, 2016 at 5:33 PM, Haggai Eran <haggaie@...lanox.com> wrote:
>>>>> +retry:
>>>>> +     spin_lock(&cg->rpool_list_lock);
>>>>> +     rpool = find_cg_rpool_locked(cg, device);
>>>>> +     if (!rpool) {
>>>>> +             spin_unlock(&cg->rpool_list_lock);
>>>>> +             ret = alloc_cg_rpool(cg, device);
>>>>> +             if (ret)
>>>>> +                     goto err;
>>>>> +             else
>>>>> +                     goto retry;
>>>> Instead of retrying after allocation of a new rpool, why not just return the
>>>> newly allocated rpool (or the existing one) from alloc_cg_rpool?
>>>
>>> It can be done, but locking semantics just becomes difficult to
>>> review/maintain with that where alloc_cg_rpool will unlock and lock
>>> conditionally later on.
>> Maybe I'm missing something, but couldn't you simply lock rpool_list_lock
>> inside alloc_cg_rpool()? It already does that around its call to
>> find_cg_rpool_locked() and the insertion to cg_list.
> 
> No. ref_count and usage counters are updated at level where lock is
> taken in charge_cg_resource().
> If I move locking rpool_list_lock inside alloc_cg_rpool, unlocking
> will continue outside, alloc_cg_rpool() when its found or allocated.
> As you acknowledged in below comment that this makes confusing to
> lock/unlock from different context, I think current implementation
> achieves both.
> (a) take lock from single context
> (b) keep functionality of find and alloc in two separate individual functions

Okay, fair enough.

>> I thought that was about functions that only locked the lock, called the
>> find function, and released the lock. What I'm suggesting is to have one
>> function that does "lock + find + allocate if needed + unlock",
> 
> I had similar function in past which does,
> "lock + find + allocate if needed + + inc_ref_cnt + unlock", (get_cg_rpool)
> update usage_counter atomically, because other thread/process might update too.
> check atomic_dec_cnt - on reaching zero, "lock + del_entry + unlock + free".
> 
> Tejun asked to simplify this to,
> 
> "lock + find + allocate if needed + inc_ref_cnt_without_atomic" + unlock".
> which I did in this patch v6.

Okay.

>> and another
>> function that does (under caller's lock) "check ref count + check max count +
>> release rpool".
> This can be done. Have one dumb basic question for thiat.
> Can we call kfree() with spin_lock held? All these years I tend to
> avoid doing so.
> 

I think so. This is an old link but I think it still applies: 
https://lkml.org/lkml/2004/11/21/130