Here goes the updated patch.
commit 74da1595eb711b77969275070cda7516bac36f5e
Signed-off-by: Sandeep K Sinha <sandeepksinha@xxxxxxxxx>
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@xxxxxxxxx> wrote:
> Neil,
>
> On Mon, Jun 15, 2009 at 4:26 AM, Neil Brown<neilb@xxxxxxx> wrote:
>>
>> Hi,
>> Thanks for this patch, and sorry for the delay in reviewing it.
>>
>> I have a few issues:
>>
>> On Saturday June 6, sandeepksinha@xxxxxxxxx wrote:
>>> Signed-off-by: Sandeep K Sinha <sandeepksinha@xxxxxxxxx>
>>>
>>> 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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