linux-kernel - Re: [PATCH 1/2] brw_mutex: big read-write mutex

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Date:	Fri, 26 Oct 2012 16:12:38 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex

On 10/26, Mikulas Patocka wrote:
>
> On Fri, 26 Oct 2012, Oleg Nesterov wrote:
>
> I didn't know about. The code is not reusable, and it doesn't really do
> locking.

That was my main point.

As for the changing fs/namespace.c to use percpu_rwsem, I am not sure
it is that simple and even worthwhile but I won't argue, I do not
pretend I understand this code.

> > I don't understand why do you both think that __mnt_want_write()
> > and mnt_make_readonly() provides the same functionality. I looked
> > at this code before I started this patch, and unless I completely
> > misread it this does very different things. It is not "lock" at all.
> >
> > Oleg.
>
> mnt_want_write uses percpu array of counters, just like percpu semaphores.

and this is all imo ;)

> The code is different, but it can be changed to use percpu rw semaphores
> (if we add percpu_down_write_trylock).

I don't really understand how you can make percpu_down_write_trylock()
atomic so that it can be called under br_write_lock(vfsmount_lock) in
sb_prepare_remount_readonly(). So I guess you also need to replace
vfsmount_lock at least. Or _trylock needs the barriers in _down_read.
Or I missed something.

Oleg.

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