linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Message-ID: <CA+55aFyBmaewJkfkBW94bSDsymg+R1wZyv064-nnj-TUrhYDyA@mail.gmail.com>
Date:	Sun, 1 Sep 2013 15:58:32 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Sedat Dilek <sedat.dilek@...il.com>,
	Waiman Long <waiman.long@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Sun, Sep 1, 2013 at 3:44 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> GRRR...  I see something else:
> void file_sb_list_del(struct file *file)
> {
>         if (!list_empty(&file->f_u.fu_list)) {
>                 lg_local_lock_cpu(&files_lglock, file_list_cpu(file));
>                 list_del_init(&file->f_u.fu_list);
>                 lg_local_unlock_cpu(&files_lglock, file_list_cpu(file));
>         }
> }
> will cheerfully cause cross-CPU traffic.  If that's what is going on, the
> earlier patch I've sent (not putting non-regulars and files opened r/o
> on ->s_list) should reduce the cacheline bouncing on that cacheline.

Hmm. That might indeed be a bad sources of cross-cpu bouncing on some
loads, but the load I test doesn't actually open any files. It just
does "stat()" on a filename.

So no "struct file *" anywhere for me..It really seems to be
vfsmount_lock itself somehow.

              Linus
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