Re: [PATCH] md linear: Protecting mddev with rcu locks to avoid races in

SandeepKsinha
2009-06-17T08:48:05Z

Hi Sujit,

Just a try to answer your question.

On Wed, Jun 17, 2009 at 1:32 PM, Sujit Karataparambil<sjt.kar@gmail.com> wrote:
> Neil,
>
> mddev is an static one time setup. Which should not have any problem
> with rcu, anyway.
> Any changes to the raid will require an rebuild using md.
>

mddev->raid_disks can change, when you add disks and also in the mean
while when you are updating the conf, the older conf might be being
used at various other places.

> Is this correct or wrong.
>
> Thanks,
> Sujit
>
> On Wed, Jun 17, 2009 at 12:16 PM, SandeepKsinha<sandeepksinha@gmail.com> wrote:
>> Here goes the updated patch.
>>
>>
>> commit 74da1595eb711b77969275070cda7516bac36f5e
>> Signed-off-by: Sandeep K Sinha <sandeepksinha@gmail.com>
>> Date:   Sat Jun 6 20:49:37 2009 +0530
>> Due to the lack of memory ordering guarantees, we may have races around
>> mddev->conf. This patch addresses the same using rcu protection to avoid
>> such race conditions.
>>
>> diff --git a/drivers/md/linear.c b/drivers/md/linear.c
>> index 9ad6ec4..a56095c 100644
>> --- a/drivers/md/linear.c
>> +++ b/drivers/md/linear.c
>> @@ -28,9 +28,11 @@
>>  static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
>>  {
>>        int lo, mid, hi;
>> -       linear_conf_t *conf = mddev_to_conf(mddev);
>> +       linear_conf_t *conf;
>>
>> +       rcu_read_lock();
>>        lo = 0;
>> +       conf = rcu_dereference(mddev->private);
>>        hi = mddev->raid_disks - 1;
>>
>>        /*
>> @@ -45,7 +47,7 @@ static inline dev_info_t *which_dev(mddev_t *mddev,
>> sector_t sector)
>>                else
>>                        lo = mid + 1;
>>        }
>> -
>> +       rcu_read_unlock();
>>        return conf->disks + lo;
>>  }
>>
>> @@ -86,36 +88,49 @@ static int linear_mergeable_bvec(struct request_queue *q,
>>  static void linear_unplug(struct request_queue *q)
>>  {
>>        mddev_t *mddev = q->queuedata;
>> -       linear_conf_t *conf = mddev_to_conf(mddev);
>> +       linear_conf_t *conf;
>>        int i;
>>
>> +       rcu_read_lock();
>> +       conf = rcu_dereference(mddev->private);
>> +
>>        for (i=0; i < mddev->raid_disks; i++) {
>>                struct request_queue *r_queue = bdev_get_queue(conf->disks[i].rdev->bdev);
>>                blk_unplug(r_queue);
>>        }
>> +       rcu_read_unlock();
>>  }
>>
>>  static int linear_congested(void *data, int bits)
>>  {
>>        mddev_t *mddev = data;
>> -       linear_conf_t *conf = mddev_to_conf(mddev);
>> +       linear_conf_t *conf;
>>        int i, ret = 0;
>>
>> +       rcu_read_lock();
>> +       conf = rcu_dereference(mddev->private);
>> +
>>        for (i = 0; i < mddev->raid_disks && !ret ; i++) {
>>                struct request_queue *q = bdev_get_queue(conf->disks[i].rdev->bdev);
>>                ret |= bdi_congested(&q->backing_dev_info, bits);
>>        }
>> +
>> +       rcu_read_unlock();
>>        return ret;
>>  }
>>
>>
>> On Wed, Jun 17, 2009 at 12:05 PM, SandeepKsinha<sandeepksinha@gmail.com> wrote:
>>> Neil,
>>>
>>> On Mon, Jun 15, 2009 at 4:26 AM, Neil Brown<neilb@suse.de> wrote:
>>>>
>>>> Hi,
>>>>  Thanks for this patch, and sorry for the delay in reviewing it.
>>>>
>>>> I have a few issues:
>>>>
>>>> On Saturday June 6, sandeepksinha@gmail.com wrote:
>>>>> Signed-off-by: Sandeep K Sinha <sandeepksinha@gmail.com>
>>>>>
>>>>>     Protecting mddev with barriers to avoid races.
>>>>
>>>> 1/ You need a lot more of an explanatory comment than this.
>>>>  At least give some hint as to what the races are.
>>>>  Give than the rcu primitives are used, it now makes sense to use
>>>>  e.g. call_rcu to free the old 'conf'.  That might reasonably be in a
>>>>  separate patch, but the comment on this patch should at least at that
>>>>  possibility.
>>>>>
>>>
>>> Sure. I shall do it for the final patch. I will also take care of this
>>> henceforth.
>>>
>>>>> diff --git a/drivers/md/linear.c b/drivers/md/linear.c
>>>>> index 9ad6ec4..a56095c 100644
>>>>> --- a/drivers/md/linear.c
>>>>> +++ b/drivers/md/linear.c
>>>>> @@ -28,9 +28,11 @@
>>>>>  static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
>>>>>  {
>>>>>       int lo, mid, hi;
>>>>> -     linear_conf_t *conf = mddev_to_conf(mddev);
>>>>> +     linear_conf_t *conf;
>>>>>
>>>>> +     rcu_read_lock();
>>>>>       lo = 0;
>>>>> +     conf = rcu_dereference(mddev->private);
>>>>>       hi = mddev->raid_disks - 1;
>>>>>
>>>>
>>>> 2/ mddev->raid_disks should really be dereferenced before 'conf'.
>>>>  Doing it the way you have done it, the 'raid_disks' value could be
>>>>  larger than the value supported by the 'conf' so things could
>>>>  go wrong.
>>>>
>>> Agreed. I hope you are referring to the case where a disk is in the
>>> process of being added to an array. Is that right ?
>>> Kindly confirm.
>>>>
>>>>>       /*
>>>>> @@ -45,7 +47,7 @@ static inline dev_info_t *which_dev(mddev_t *mddev,
>>>>> sector_t sector)
>>>>>               else
>>>>>                       lo = mid + 1;
>>>>>       }
>>>>> -
>>>>> +     rcu_read_unlock();
>>>>>       return conf->disks + lo;
>>>>>  }
>>>>
>>>> 3/ We are accessing conf->disks well after the rcu_lock has been released.
>>>>   That is not exactly a problem with the code as it stands.  But if
>>>>   we do go ahead and free the old 'conf' with call_rcu, then this
>>>>   becomes racy.
>>>>   We should hold the rcu_read_lock for the entire time that we are
>>>>   accessing the contents of 'conf'.
>>>>
>>> True.
>>>
>>>>   That means we don't take rcu_read_lock in which_dev, but rather
>>>>   take it in the two functions that call which_dev.
>>>>
>>>
>>> I have made that change.
>>>>>
>>>>> @@ -86,36 +88,49 @@ static int linear_mergeable_bvec(struct request_queue *q,
>>>>>  static void linear_unplug(struct request_queue *q)
>>>>>  {
>>>>>       mddev_t *mddev = q->queuedata;
>>>>> -     linear_conf_t *conf = mddev_to_conf(mddev);
>>>>> +     linear_conf_t *conf;
>>>>>       int i;
>>>>>
>>>>> +     rcu_read_lock();
>>>>> +     conf = rcu_dereference(mddev->private);
>>>>> +
>>>>>       for (i=0; i < mddev->raid_disks; i++) {
>>>>>               struct request_queue *r_queue = bdev_get_queue(conf->disks[i].rdev->bdev);
>>>>>               blk_unplug(r_queue);
>>>>>       }
>>>>> +     rcu_read_unlock();
>>>>>  }
>>>>>
>>>>>  static int linear_congested(void *data, int bits)
>>>>>  {
>>>>>       mddev_t *mddev = data;
>>>>> -     linear_conf_t *conf = mddev_to_conf(mddev);
>>>>> +     linear_conf_t *conf;
>>>>>       int i, ret = 0;
>>>>>
>>>>> +     rcu_read_lock();
>>>>> +     conf = rcu_dereference(mddev->private);
>>>>> +
>>>>>       for (i = 0; i < mddev->raid_disks && !ret ; i++) {
>>>>>               struct request_queue *q = bdev_get_queue(conf->disks[i].rdev->bdev);
>>>>>               ret |= bdi_congested(&q->backing_dev_info, bits);
>>>>>       }
>>>>> +
>>>>> +     rcu_read_unlock();
>>>>>       return ret;
>>>>>  }
>>>>>
>>>>>  static sector_t linear_size(mddev_t *mddev, sector_t sectors, int raid_disks)
>>>>>  {
>>>>> -     linear_conf_t *conf = mddev_to_conf(mddev);
>>>>> -
>>>>> +     linear_conf_t *conf;
>>>>> +     sector_t array_sectors;
>>>>> +     rcu_read_lock();
>>>>> +     conf = rcu_dereference(mddev->private);
>>>>>       WARN_ONCE(sectors || raid_disks,
>>>>>                 "%s does not support generic reshape\n", __func__);
>>>>> -
>>>>> -     return conf->array_sectors;
>>>>> +     array_sectors = conf->array_sectors;
>>>>> +     rcu_read_unlock();
>>>>> +
>>>>> +     return array_sectors;
>>>>>  }
>>>>>
>>>>>  static linear_conf_t *linear_conf(mddev_t *mddev, int raid_disks)
>>>>> @@ -215,15 +230,14 @@ static int linear_add(mddev_t *mddev, mdk_rdev_t *rdev)
>>>>>               return -EINVAL;
>>>>>
>>>>>       rdev->raid_disk = rdev->saved_raid_disk;
>>>>> -
>>>>> -     newconf = linear_conf(mddev,mddev->raid_disks+1);
>>>>> +     newconf = linear_conf(mddev,mddev->raid_disks + 1);
>>>>>
>>>>>       if (!newconf)
>>>>>               return -ENOMEM;
>>>>>
>>>>>       newconf->prev = mddev_to_conf(mddev);
>>>>> -     mddev->private = newconf;
>>>>>       mddev->raid_disks++;
>>>>> +     rcu_assign_pointer(mddev->private,newconf);
>>>>>       md_set_array_sectors(mddev, linear_size(mddev, 0, 0));
>>>>>       set_capacity(mddev->gendisk, mddev->array_sectors);
>>>>>       return 0;
>>>>> @@ -231,14 +245,17 @@ static int linear_add(mddev_t *mddev, mdk_rdev_t *rdev)
>>>>>
>>>>>  static int linear_stop (mddev_t *mddev)
>>>>>  {
>>>>> -     linear_conf_t *conf = mddev_to_conf(mddev);
>>>>> -
>>>>> +     linear_conf_t *conf;
>>>>> +
>>>>> +     rcu_read_lock();
>>>>> +     conf = rcu_dereference(mddev->private);
>>>>>       blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
>>>>>       do {
>>>>>               linear_conf_t *t = conf->prev;
>>>>>               kfree(conf);
>>>>>               conf = t;
>>>>>       } while (conf);
>>>>> +     rcu_read_unlock();
>>>>>
>>>>
>>>> 4/ We don't need the rcu protection here as we hold ->reconfig_mutex
>>>>   both in linear_add and linear_stop, so they cannot race.
>>>>   Adding a comment to this effect might be a good idea though.
>>>>
>>>
>>> Fine. Shall do this as well.
>>>
>>> The new patch will follow soon.
>>>
>>>> Thanks,
>>>>
>>>> NeilBrown
>>>>
>>>>
>>>>>       return 0;
>>>>>  }
>>>>> --
>>>>> Regards,
>>>>> Sandeep.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> “To learn is to change. Education is a process that changes the learner.”
>>>>
>>>
>>>
>>>
>>> --
>>> Regards,
>>> Sandeep.
>>>
>>>
>>>
>>>
>>>
>>>
>>> “To learn is to change. Education is a process that changes the learner.”
>>>
>>
>>
>>
>> --
>> Regards,
>> Sandeep.
>>
>>
>>
>>
>>
>>
>> “To learn is to change. Education is a process that changes the learner.”
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>
>
>
> --
> -- Sujit K M
>

--
Regards,
Sandeep.

“To learn is to change. Education is a process that changes the learner.”