On Thu, Dec 31, 2009 at 02:02:38AM +0800, Andi Kleen wrote:
> 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?
Oh I pretended that the problem don't exist..
To be serious, the race only exist inside a mutithread application,
where one single fd is shared between two threads, one is doing
fadvise, another doing readahead.
A sane application won't do fadvise(POSIX_FADV_RANDOM) while active
reads are going one concurrently: this leads to indeterminate behavior
by itself -- from which request the random hint takes effect?
fadvise() shall always be in the same streamline with all reads.
In real workloads, 1% applications may do POSIX_FADV_RANDOM, among
which 1% applications may be broken. And if the race does happen, the
impact is very small. So I choose to just ignore the race and use
non-atomic operations..
Thanks,
Fengguang
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