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

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Date:	Thu, 30 Jul 2015 22:16:56 -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 07/29/2015 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.

Cheers,
Longman


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