Re: [PATCH 17/19] fs: buffer: Do not use unnecessary atomic operations when discarding buffers

[Peter Zijlstra]

On Tue, May 13, 2014 at 10:45:48AM +0100, Mel Gorman wrote:
> Discarding buffers uses a bunch of atomic operations when discarding buffers
> because ...... I can't think of a reason. Use a cmpxchg loop to clear all the
> necessary flags. In most (all?) cases this will be a single atomic operations.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
>  fs/buffer.c                 | 14 +++++++++-----
>  include/linux/buffer_head.h |  5 +++++
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 9ddb9fc..e80012d 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1485,14 +1485,18 @@ EXPORT_SYMBOL(set_bh_page);
>   */
>  static void discard_buffer(struct buffer_head * bh)
>  {
> +	unsigned long b_state, b_state_old;
> +
>  	lock_buffer(bh);
>  	clear_buffer_dirty(bh);
>  	bh->b_bdev = NULL;
> -	clear_buffer_mapped(bh);
> -	clear_buffer_req(bh);
> -	clear_buffer_new(bh);
> -	clear_buffer_delay(bh);
> -	clear_buffer_unwritten(bh);
> +	b_state = bh->b_state;
> +	for (;;) {
> +		b_state_old = cmpxchg(&bh->b_state, b_state, (b_state & ~BUFFER_FLAGS_DISCARD));
> +		if (b_state_old == b_state)
> +			break;
> +		b_state = b_state_old;
> +	}
>  	unlock_buffer(bh);
>  }

So.. I'm soon going to introduce atomic_{or,and}() and
atomic64_{or,and}() across the board, but of course this isn't an
atomic_long_t but a regular unsigned long.

Its a bit unfortunate we have this discrepancy with types vs atomic ops,
there's:

  cmpxchg, xchg -- mostly available for all 1,2,3,4 (and 8 where
  appropriate) byte values.

  bitops -- operate on unsigned long *

  atomic* -- operate on atomic_*t


So while ideally we'd be able to use the unconditional atomic and
operation which is available on a lot of architectures, we'll be stuck
with a cmpxchg loop instead :/

*sigh*

Anyway, nothing wrong with this patch, however, you could, if you really
wanted to push things, also include BH_Lock in that clear :-)

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