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Date: Mon, 06 Sep 2010 09:43:32 +0200
From: walter harms <wharms@....de>
To: Neil Brown <neilb@...e.de>
CC: Sam Ravnborg <sam@...nborg.org>,
Kulikov Vasiliy <segooon@...il.com>,
kernel-janitors@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>,
linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md: do not use ++ in rcu_dereference() argument
Neil Brown schrieb:
> On Sun, 5 Sep 2010 22:39:08 +0200
> Sam Ravnborg <sam@...nborg.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@...il.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@...e.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@...il.com>
> Signed-off-by: NeilBrown <neilb@...e.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;
> -
> /* 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;
i think it is more readable to use:
rdev = rcu_dereference(conf->mirrors[disk].rdev);
if ()
> + 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;
> - 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;
again i would set
rdev=rcu_dereference(conf->mirrors[disk].rdev));
before if() like it was in the original the statement is complex
anything that reduces the complexity is good.
>
> 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)
>
just my 2 cents,
re,
wh
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