linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Message-ID: <CA+55aFy8ym1KSFv753wGrYn8qG5eK6YOy_nOCpYiqPjoLJwysg@mail.gmail.com>
Date:	Wed, 8 Feb 2012 08:34:53 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Shaohua Li <shaohua.li@...el.com>, Jens Axboe <axboe@...nel.dk>,
	Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [PATCH] block: strip out locking optimization in put_io_context()

On Wed, Feb 8, 2012 at 8:29 AM, Tejun Heo <tj@...nel.org> wrote:
>
> Can you please try the following one?  Thanks a lot!

If you can use it as a rwlock, why can't you do it with RCU?

Usually rwlocks are a bad idea. They tend to be more expensive than
spinlocks, and the extra parallelism is almost never noticeable
(except as "more cacheline bounces") for something that is appropriate
for a non-sleeping lock.

There's a *very* few situations where rwlock is the right thing, but
it really almost always is a horribly bad idea.

                     Linus
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