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

SandeepKsinha
2009-06-17T06:46:51Z

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.”