On Tue 13-05-14 13:50:07, Mel Gorman wrote:
> On Tue, May 13, 2014 at 01:09:51PM +0200, Peter Zijlstra wrote:
> > 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
>
> I hit the same problem when dealing with pageblock bitmap. I would have
> preferred it to do an atomic_read() but the actual conversion to use
> atomic_t for the map became a mess with little or no upside.
>
> >
> > 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 :-)
>
> That's a bold strategy Cotton.
>
> Untested patch on top
Although this looks correct, I have to say I prefer the explicit
unlock_buffer() unless this has a measurable benefit.
Honza
> ---8<---
> diff --git a/fs/buffer.c b/fs/buffer.c
> index e80012d..42fcb6d 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1490,6 +1490,8 @@ static void discard_buffer(struct buffer_head * bh)
> lock_buffer(bh);
> clear_buffer_dirty(bh);
> bh->b_bdev = NULL;
> +
> + smp_mb__before_clear_bit();
> b_state = bh->b_state;
> for (;;) {
> b_state_old = cmpxchg(&bh->b_state, b_state, (b_state & ~BUFFER_FLAGS_DISCARD));
> @@ -1497,7 +1499,13 @@ static void discard_buffer(struct buffer_head * bh)
> break;
> b_state = b_state_old;
> }
> - unlock_buffer(bh);
> +
> + /*
> + * BUFFER_FLAGS_DISCARD include BH_lock so it has been cleared so the
> + * wake_up_bit is the last part of a unlock_buffer
> + */
> + smp_mb__after_clear_bit();
> + wake_up_bit(&bh->b_state, BH_Lock);
> }
>
> /**
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 95f565a..523db58 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -80,7 +80,7 @@ struct 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)
> + 1 << BH_Delay | 1 << BH_Unwritten | 1 << BH_Lock)
>
> /*
> * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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