linux-kernel - Re: [PATCH] proc: change proc_subdir

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Message-ID: <87wpxcdzbr.fsf@x220.int.ebiederm.org>
Date:	Mon, 03 Aug 2015 13:03:20 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Waiman Long <waiman.long@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH] proc: change proc_subdir_lock to a rwlock

Waiman Long <waiman.long@...com> writes:

> On 07/30/2015 10:16 PM, Waiman Long wrote:
>> On 07/29/2015 06:21 PM, Eric W. Biederman wrote:
>>> Two quick questions.
>>>
>>> - What motivates this work?  Are you seeing lots of
>>>    parallel reads on proc?
>>
>> The micro-benchmark that I used was artificial, but it was used to reproduce
>> an exit hanging problem that I saw in real application. In fact, only allow
>> one task to do a lookup seems too limiting to me.
>>> - Why not rcu?  Additions and removal of proc generic
>>>    files is very rare.  Conversion to rcu for reads should
>>>    perform better and not take much more work.
>>
>> RCU is harder to verify its correctness, whereas rwlock is easier to use and
>> understand. If it is really a performance critical path where every extra bit
>> of performance counts, I will certainly think RCU may be the right
>> choice. However, in this particular case, I don't think using RCU will give
>> any noticeable performance gain compared with a rwlock.
>
> One more thing, RCU is typically used with linked list. It is not easy to use
> RCU with rbtree and may require major changes to the code.
>
> Another alternative is to use seqlock + RCU, but it will still need more code
> changes than rwlock.

I had forgotten we had switch proc directories to rbtrees.   So on that
note.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Eric
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