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

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Date:	Fri, 30 Aug 2013 21:26:08 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Waiman Long <waiman.long@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Fri, Aug 30, 2013 at 03:52:49PM -0400, Waiman Long wrote:

> So it is the mnput_no_expire() function that is doing all the
> lg_global_lock() calls.

Interesting...  So you are getting a lot of mntput() with ->mnt_ns being
NULL?  I wonder which type it is...  Note that anything mounted will
have non-NULL ->mnt_ns until umount and anything obtained via
kern_mount/kern_mount_data will also have a non-NULL ->mnt_ns - until
kern_unmount().

Could you try to gather stats of that sort?  Normally that path should
be only hit by fput() when we have a lazy-unmounted fs and close an opened
file on it...

I see one potential stupidity in that area (simple_pin_fs() ought to set
->mnt_ns, with simple_release_fs() clearing it), but there's not a lot
of fs types that would use the damn thing...
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