Re: [PATCH 1/2] mm: Remove definition of clear_bit_unlock_is_negative_byte

[Will Deacon <will@kernel.org>]

Hi Matthew,

Sorry I missed this, I'm over on @kernel.org now and don't have access to
my old @arm.com address anymore.

On Thu, Mar 26, 2020 at 05:24:28AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> This local definition hasn't been used since commit 84c6591103db
> ("locking/atomics, asm-generic/bitops/lock.h: Rewrite using
> atomic_fetch_*()") which provided a default definition.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> ---
>  mm/filemap.c | 23 -----------------------
>  1 file changed, 23 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 80f7e1ae744c..312afbfcb49a 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1248,29 +1248,6 @@ void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter)
>  }
>  EXPORT_SYMBOL_GPL(add_page_wait_queue);
>  
> -#ifndef clear_bit_unlock_is_negative_byte
> -
> -/*
> - * PG_waiters is the high bit in the same byte as PG_lock.
> - *
> - * On x86 (and on many other architectures), we can clear PG_lock and
> - * test the sign bit at the same time. But if the architecture does
> - * not support that special operation, we just do this all by hand
> - * instead.
> - *
> - * The read of PG_waiters has to be after (or concurrently with) PG_locked
> - * being cleared, but a memory barrier should be unneccssary since it is
> - * in the same byte as PG_locked.
> - */
> -static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem)
> -{
> -	clear_bit_unlock(nr, mem);
> -	/* smp_mb__after_atomic(); */
> -	return test_bit(PG_waiters, mem);
> -}
> -
> -#endif
> -

I'd really like to do this, but I worry that the generic definition still
isn't available on all architectures depending on how they pull together
their bitops.h. Have you tried building for alpha or s390? At a quick
glance, they look like they might fall apart :(

Will