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

Neil Brown
2010-09-06T05:29:31Z

On Sun, 5 Sep 2010 22:39:08 +0200
Sam Ravnborg <sam@ravnborg.org> wrote:

> On Sun, Sep 05, 2010 at 11:23:35PM +0400, Kulikov Vasiliy wrote:
> > On Sun, Sep 05, 2010 at 21:01 +0200, Sam Ravnborg wrote:
> > > 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
> > >
> > > 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.
> >
> > No, the original code increments new_disk and then dereferences mirrors.
> >
> > The full code:
> >
> > 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;
> > }
> > }
> >
> > so,
> >
> > for (a; b; c = f(++g)) {
> > ...
> > }
>
> Thanks - that explains it.
> This code really screams for a helper function but thats another matter.

Not an uncommon complaint about my code as it happens......

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;
}

-
/* 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;
+
+ if (r1_bio->bios[disk] = IO_BLOCKED
+ || !(rdev = rcu_dereference(conf->mirrors[disk].rdev))
+ || !test_bit(In_sync, &rdev->flags))
+ continue;
+
+ if (test_bit(WriteMostly, &rdev->flags)) {
+ new_disk = disk;
+ continue;
}
+ new_disk = disk;
+ break;
}

- if (new_disk < 0)
+ if (new_disk < 0 || choose_first)
goto rb_out;

- disk = new_disk;
- /* now disk = new_disk = starting point for search */
-
/*
* Don't change to another disk for sequential reads:
*/
@@ -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;

- 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;

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)