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

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Date:	Thu, 30 Jul 2015 22:53:17 -0400
From:	Waiman Long <waiman.long@...com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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

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.

Cheers,
Longman
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