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

Sam Ravnborg
2010-09-06T19:21:28Z

> Comments?

Looks better but can still use a few improvements.
See below.

Sam
>
> 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;

To increase readability the general recommendation is:
1) Sort variable definitions with the longest first.
2) Do not assing variables when they are defined, do that on a separate line
below the variable definitions.
With one empty line after variable definitions.

>
> 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;
> }
>
> -
> /* make sure the disk is operational */
> - 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 <= 0)
> - new_disk = conf->raid_disks;
> - new_disk--;
> - if (new_disk = disk) {
> - new_disk = wonly_disk;
> - break;
> + for (i = 0 ; i < conf->raid_disks ; i++) {
> + int disk = start_disk + i;
> + if (disk >= conf->raid_disks)
> + disk -= conf->raid_disks;
1) Please comment on the purpose of the for loop
2) See comments above aboyt variable definitions

> +
> + if (r1_bio->bios[disk] = IO_BLOCKED
> + || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
> + || !test_bit(In_sync, &rdev->flags))
The rather complex expression - which includes a well hidden assignment -
is repeated a few lines later.
Please use a helper function and do not use such hidden assignments.

> + continue;
> +
> + if (test_bit(WriteMostly, &rdev->flags)) {
> + new_disk = disk;
> + continue;
> }
> + new_disk = disk;
> + break;
> }
>
> @@ -491,20 +474,20 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> if (this_sector = conf->mirrors[new_disk].head_position)
> goto rb_out;
>
> - current_distance = abs(this_sector - conf->mirrors[disk].head_position);
> + current_distance = abs(this_sector
> + - conf->mirrors[new_disk].head_position);
>
> - /* Find the disk whose head is closest */
> + /* look for a better disk - i.e. head is closer */
> + start_disk = new_disk;
> + for (i = 1; i < conf->raid_disks; i++) {
> + int disk = start_disk + 1;
> + if (disk >= conf->raid_disks)
> + disk -= conf->raid_disks;
See comments about for loop above.
I also cannot see why we suddenly start with 1 where the other
almost identical for loop starts with 0?
If I wonder then someone else will wonder too => comment please.

>
> - do {
> - if (disk <= 0)
> - disk = conf->raid_disks;
> - disk--;
> -
> - rdev = rcu_dereference(conf->mirrors[disk].rdev);
> -
> - if (!rdev || r1_bio->bios[disk] = IO_BLOCKED ||
> - !test_bit(In_sync, &rdev->flags) ||
> - test_bit(WriteMostly, &rdev->flags))
> + if (r1_bio->bios[disk] = IO_BLOCKED
> + || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
> + || !test_bit(In_sync, &rdev->flags)
> + || test_bit(WriteMostly, &rdev->flags))
> continue;
Here the complex expression is repeated - at least almost identical.

>
> if (!atomic_read(&rdev->nr_pending)) {
> @@ -516,11 +499,9 @@ static int read_balance(conf_t *conf, r1bio_t *r1_bio)
> current_distance = new_distance;
> new_disk = disk;
> }
> - } while (disk != conf->last_used);
> + }
>
> rb_out:
> -
> -
> if (new_disk >= 0) {
> rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
> if (!rdev)
>

Sam