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

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Date:	Mon, 17 Dec 2012 18:42:44 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	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 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.  (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.

--Andy
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