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

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Date:	Fri, 26 Oct 2012 09:22:23 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Oleg Nesterov <oleg@...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 Fri, 26 Oct 2012, Oleg Nesterov wrote:

> On 10/26, Dave Chinner wrote:
> >
> > On Thu, Oct 25, 2012 at 10:09:31AM -0400, Mikulas Patocka wrote:
> > >
> > > Yes, mnt_want_write()/mnt_make_readonly() do the same thing as percpu rw
> > > semaphores. I think you can convert mnt_want_write()/mnt_make_readonly()
> > > to use percpu rw semaphores and remove the duplicated code.
> >
> > I think you misunderstood my point - that rather than re-inventing
> > the wheel, why didn't you just copy something that is known to
> > work?

I didn't know about. The code is not reusable, and it doesn't really do 
locking. And it has two barriers on the read path, while percpu rw 
semaphores have none.

> 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.

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

__mnt_want_write could call percpu_down_read and check if it is readonly 
(if it is, drop the lock and return -EROFS)
__mnt_drop_write could call percpu_up_read
mnt_make_readonly and sb_prepare_remount_readonly could call 
percpu_down_write_trylock instead of mnt_get_writers (if they get the 
write lock, set it to readonly and drop the write lock)

... and that's it, then, you can remove MNT_WRITE_HOLD, the barriers, 
spinning and other complexity from fs/namespace.c

Mikulas
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