Re: [PATCH] md: do not use ++ in rcu_dereference() argument

Sam Ravnborg
2010-09-05T19:01:39Z

On Sun, Sep 05, 2010 at 10:32:18PM +0400, Kulikov Vasiliy wrote:
> From: Vasiliy Kulikov <segooon@gmail.com>
>
> rcu_dereference() is macro, so it might use its argument twice.
> Argument must not has side effects.
>
> It was found by compiler warning:
> drivers/md/raid1.c: In function ‘read_balance’:
> drivers/md/raid1.c:445: warning: operation on ‘new_disk’ may be undefined
>
> Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
> ---
> drivers/md/raid1.c | 3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> index ad83a4d..12194df 100644
> --- a/drivers/md/raid1.c
> +++ b/drivers/md/raid1.c
> @@ -442,7 +442,7 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> r1_bio->bios[new_disk] = IO_BLOCKED ||
> !rdev || !test_bit(In_sync, &rdev->flags)
> || test_bit(WriteMostly, &rdev->flags);
> - rdev = rcu_dereference(conf->mirrors[++new_disk].rdev)) {
> + rdev = rcu_dereference(conf->mirrors[new_disk].rdev)) {
>
> if (rdev && test_bit(In_sync, &rdev->flags) &&
> r1_bio->bios[new_disk] != IO_BLOCKED)
> @@ -452,6 +452,7 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> new_disk = wonly_disk;
> break;
> }
> + new_disk++;
> }
> goto rb_out;

This change looks wrong.
In the original implementation new_disk is incremented and
then we do the array lookup.
With your implementation it looks like we increment it after
the array lookup.

Sam