Re: [RFC][PATCH 2/2] readahead: avoid page-by-page reads on POSIX_FADV_RANDOM

[Andi Kleen]

Wu Fengguang <fengguang.wu@xxxxxxxxx> writes:
>   * the ra fields can be accessed concurrently in a racy way.
> --- linux.orig/mm/fadvise.c	2009-12-30 13:02:03.000000000 +0800
> +++ linux/mm/fadvise.c	2009-12-30 13:23:05.000000000 +0800
> @@ -77,12 +77,14 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, lof
>  	switch (advice) {
>  	case POSIX_FADV_NORMAL:
>  		file->f_ra.ra_pages = bdi->ra_pages;
> +		file->f_ra.flags &= ~RA_FLAG_RANDOM;
>  		break;
>  	case POSIX_FADV_RANDOM:
> -		file->f_ra.ra_pages = 0;
> +		file->f_ra.flags |= RA_FLAG_RANDOM;

What prevents this from racing with a parallel readahead
state modification, losing the bits?

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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