On Tue, May 13, 2014 at 03:29:00PM -0700, Andrew Morton wrote:
> On Tue, 13 May 2014 10:45:48 +0100 Mel Gorman <mgorman@xxxxxxx> 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.
> >
> > --- 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);
> > }
> >
> > --- a/include/linux/buffer_head.h
> > +++ b/include/linux/buffer_head.h
> > @@ -77,6 +77,11 @@ struct buffer_head {
> > atomic_t b_count; /* users using this buffer_head */
> > };
> >
> > +/* Bits that are cleared during an invalidate */
> > +#define BUFFER_FLAGS_DISCARD \
> > + (1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
> > + 1 << BH_Delay | 1 << BH_Unwritten)
> > +
>
> There isn't much point in having this in the header file is there?
>
No, it's not necessary. I was just keeping it with the definition of the
flags. Your fix on top looks fine.
--
Mel Gorman
SUSE Labs
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