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

Neil Brown
2010-09-06T11:05:43Z

On Mon, 06 Sep 2010 09:43:32 +0200
walter harms <wharms@bfs.de> wrote:

>
>
> Neil Brown schrieb:
> > I've taken the opportunity to substantially re-write that code.
> >
> > Comments?
> >
> > Thanks,
> > NeilBrown
> >
> > commit e4062735c8f7233923df5858ed20f1278f3ee669
> > Author: NeilBrown <neilb@suse.de>
> > Date: Mon Sep 6 14:10:08 2010 +1000
> >
> > md: tidy up device searches in read_balance.
> >
> > We have a pre-increment side-effect in the arg to a macro:
> > rcu_dereference
> >
> > This is poor form and triggers a warning. Rather than just fix that,
> > take the opportunity to re-write the code it make it more readable.
> >
> > Reported-by: Kulikov Vasiliy <segooon@gmail.com>
> > Signed-off-by: NeilBrown <neilb@suse.de>
> >
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index ad83a4d..e29e13f 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -420,11 +420,13 @@ static void raid1_end_write_request(struct bio *bio, int error)
> > static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> > {
> > const sector_t this_sector = r1_bio->sector;
> > - int new_disk = conf->last_used, disk = new_disk;
> > - int wonly_disk = -1;
> > + int new_disk = -1;
> > + int start_disk;
> > + int i;
> > const int sectors = r1_bio->sectors;
> > sector_t new_distance, current_distance;
> > mdk_rdev_t *rdev;
> > + int choose_first;
> >
> > rcu_read_lock();
> > /*
> > @@ -435,54 +437,35 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> > retry:
> > if (conf->mddev->recovery_cp < MaxSector &&
> > (this_sector + sectors >= conf->next_resync)) {
> > - /* Choose the first operational device, for consistancy */
> > - new_disk = 0;
> > -
> > - for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
> > - 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)) {
> > -
> > - if (rdev && test_bit(In_sync, &rdev->flags) &&
> > - r1_bio->bios[new_disk] != IO_BLOCKED)
> > - wonly_disk = new_disk;
> > -
> > - if (new_disk = conf->raid_disks - 1) {
> > - new_disk = wonly_disk;
> > - break;
> > - }
> > - }
> > - goto rb_out;
> > + choose_first = 1;
> > + start_disk = 0;
> > + } else {
> > + choose_first = 0;
> > + start_disk = conf->last_used;
> > }
> >
>
>
> perhaps you can drop the else when you init with
> choose_first = 0;
> start_disk = conf->last_used;

Perhaps. Though given the 'retry' loop it isn't obvious that would do the
right thing.
I think I'll keep this bit as-is. I think it helps see to two cases more
clearly.

> > + if (r1_bio->bios[disk] = IO_BLOCKED
> > + || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
> > + || !test_bit(In_sync, &rdev->flags))
> > + continue;
>
> i think it is more readable to use:
>
> rdev = rcu_dereference(conf->mirrors[disk].rdev);
> if ()
>

I think assignments inside 'if' statements have their place, but it seems
that this is far from universal. I've made this change, thanks.

>
>
> > + if (test_bit(WriteMostly, &rdev->flags)) {
> > + new_disk = disk;
> > + continue;
> > }
> > + new_disk = disk;
> > + break;
> > }
>
> to improve readability:
>
> new_disk = disk;
> if ( ! test_bit(WriteMostly, &rdev->flags) )
> break;

Yes, that is a distinct improvement. I've made that change too.

Thanks a lot for your review!!

NeilBrown