linux-kernel - Re: [PATCH 0/2 v2] dcache: get/release read lock in read_seqbegin_or

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Date:	Thu, 12 Sep 2013 10:30:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 0/2 v2] dcache: get/release read lock in
 read_seqbegin_or_lock() & friend

On Thu, Sep 12, 2013 at 9:38 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Sep 12, 2013 at 7:55 AM, Waiman Long <Waiman.Long@...com> wrote:
>> Change log
>> ----------
>> v1->v2:
>>   - Rename the new seqlock primitives to read_seqexcl_lock* and
>>     read_seqexcl_unlock*.
>
> Applied.

Btw, when I tried to benchmark this, I failed miserably.

Why?

If you do a threaded benchmark of "getcwd()", you end up spending all
your time in a spinlock anyway: get_fs_root_and_pwd() takes the
fs->lock to get the root/pwd.

Now, AIM7 probably uses processes, not threads, so you don't see this,
and maybe I shouldn't care. But looking at it, it annoys me
enormously, because the whole get_fs_root_and_pwd() is just stupid.

Putting it all under the RCU lock and then changing it to use
get_fs_root_and_pwd_rcu() that just uses the fs->seq sequence
read-lock looks absolutely trivial.

              Linus
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