linux-kernel - Re: Are there u32 atomic bitops? (or dealing w/ i

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Message-ID: <20121218213043.GF15182@dastard>
Date:	Wed, 19 Dec 2012 08:30:43 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: Are there u32 atomic bitops? (or dealing w/ i_flags)

On Mon, Dec 17, 2012 at 06:42:44PM -0800, Andy Lutomirski wrote:
> On Mon, Dec 17, 2012 at 5:57 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Mon, Dec 17, 2012 at 05:10:21PM -0800, Andy Lutomirski wrote:
> >> I want to change inode->i_flags access to be atomic -- there are some
> >> locking oddities right now, I think, and I want to use a new inode
> >> flag to signal mtime updates from page_mkwrite.  The problem is that
> >> i_flags is an unsigned int, and making it an unsigned long seems like
> >> a waste, but there aren't any u32 atomic bitops.
> >
> > ... and atomic accesses cost more.  A lot more on some architectures.
> > FWIW, atomic_t *is* 32bit on 32bit architectures, which still doesn't
> > make it a good idea.
> 
> Are atomic_set_mask and atomic_clear_mask as fast as set_bit and
> friends on all archs?
> 
> In any case, i_flags looks like it's rarely written, so I find it a
> bit hard to believe that making it atomic would hurt.  Isn't
> atomic_read equivalent to non-atomic reads everywhere?
> 
> I want page_mkwrite to set a flag (without taking i_mutex) but *not*
> call file_update_time and then to have the writeback paths update the
> inode time.

Deadlocks ahoy!

We don't currently take the i_mutex anywhere in the writeback path
as the writeback path is nested inside the i_mutex. Hence this seems
like an extremely dangerous thing to do...

> (This, along with stable pages, is the major cause of
> long sleeps in my application.)  OTOH, maybe I should just use i_state
> and i_lock for this.

Or, perhaps, export O_CMTIME to fcntl and/or open?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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