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SubjectRe: [PATCH] block: fix accounting bug on cross partition merges
On 12/17/2010 08:06 PM, Jens Axboe wrote:
> On 2010-12-17 14:42, Jerome Marchand wrote:
>>
>> /proc/diskstats would display a strange output as follows.
>
> [snip]
>
> This looks a lot better! One comment:
>
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 4ce953f..064921d 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -64,13 +64,16 @@ static void drive_stat_acct(struct request *rq, int new_io)
>> return;
>>
>> cpu = part_stat_lock();
>> - part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>>
>> - if (!new_io)
>> + if (!new_io) {
>> + part = rq->part;
>> part_stat_inc(cpu, part, merges[rw]);
>> - else {
>> + } else {
>> + part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>> part_round_stats(cpu, part);
>> part_inc_in_flight(part, rw);
>> + kref_get(&part->ref);
>> + rq->part = part;
>> }
>>
>> part_stat_unlock();
>
> I don't think this is completely safe. The rcu lock is held due to the
> part_stat_lock(), but that only prevents the __delete_partition()
> callback from happening. Lets say you have this:
>
> CPU0 CPU1
> part = disk_map_sector_rcu()
> kref_put(part); <- now 0

A kref_get(part) happens here, or am I missing something?
If we use an atomic kref_test_and_get() function, which only increment the
refcount if it's not zero, we would be safe. If kref_test_and_get() fails,
we can cache rq->rq_disk->part0 instead. See my drafty patch below.

> part_stat_unlock()
> __delete_partition();
> ...
> delete_partition_rcu_cb();
> merge, or endio, boom
>
> Now rq has ->part pointing to freed memory, later merges or end
> accounting will touch freed memory.
>
> I think we can fix this by just having delete_partition_rcu_rb() check
> the reference count and return if non-zero. Since someone holds a
> reference to the table, they will drop it and we'll re-schedule the rcu
> callback.
>
>


---
diff --git a/block/blk-core.c b/block/blk-core.c
index 4ce953f..72d12d2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -64,13 +64,21 @@ static void drive_stat_acct(struct request *rq, int new_io)
return;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));

- if (!new_io)
+ if (!new_io) {
+ part = rq->part;
part_stat_inc(cpu, part, merges[rw]);
- else {
+ } else {
+ part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
+ if(part->partno && !kref_test_and_get(&part->ref))
+ /*
+ * The partition is already being removed,
+ * the request will be accounted on the disk only
+ */
+ part = &rq->rq_disk->part0;
part_round_stats(cpu, part);
part_inc_in_flight(part, rw);
+ rq->part = part;
}

part_stat_unlock();
@@ -128,6 +136,7 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
rq->ref_count = 1;
rq->start_time = jiffies;
set_start_time_ns(rq);
+ rq->part = NULL;
}
EXPORT_SYMBOL(blk_rq_init);

@@ -1776,7 +1785,7 @@ static void blk_account_io_completion(struct request *req, unsigned int bytes)
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = req->part;
part_stat_add(cpu, part, sectors[rw], bytes >> 9);
part_stat_unlock();
}
@@ -1796,13 +1805,15 @@ static void blk_account_io_done(struct request *req)
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = req->part;

part_stat_inc(cpu, part, ios[rw]);
part_stat_add(cpu, part, ticks[rw], duration);
part_round_stats(cpu, part);
part_dec_in_flight(part, rw);

+ if (part->partno)
+ kref_put(&part->ref, __delete_partition);
part_stat_unlock();
}
}
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 77b7c26..3334578 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -351,11 +351,13 @@ static void blk_account_io_merge(struct request *req)
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = req->part;

part_round_stats(cpu, part);
part_dec_in_flight(part, rq_data_dir(req));

+ if (part->partno)
+ kref_put(&part->ref, __delete_partition);
part_stat_unlock();
}
}
diff --git a/block/genhd.c b/block/genhd.c
index 5fa2b44..0c55eae 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1192,6 +1192,7 @@ struct gendisk *alloc_disk_node(int minors, int node_id)
return NULL;
}
disk->part_tbl->part[0] = &disk->part0;
+ kref_init(&disk->part0.ref);

disk->minors = minors;
rand_initialize_disk(disk);
diff --git a/fs/partitions/check.c b/fs/partitions/check.c
index 0a8b0ad..0123717 100644
--- a/fs/partitions/check.c
+++ b/fs/partitions/check.c
@@ -372,6 +372,13 @@ static void delete_partition_rcu_cb(struct rcu_head *head)
put_device(part_to_dev(part));
}

+void __delete_partition(struct kref *ref)
+{
+ struct hd_struct *part = container_of(ref, struct hd_struct, ref);
+
+ call_rcu(&part->rcu_head, delete_partition_rcu_cb);
+}
+
void delete_partition(struct gendisk *disk, int partno)
{
struct disk_part_tbl *ptbl = disk->part_tbl;
@@ -390,7 +397,7 @@ void delete_partition(struct gendisk *disk, int partno)
kobject_put(part->holder_dir);
device_del(part_to_dev(part));

- call_rcu(&part->rcu_head, delete_partition_rcu_cb);
+ kref_put(&part->ref, __delete_partition);
}

static ssize_t whole_disk_show(struct device *dev,
@@ -489,6 +496,7 @@ struct hd_struct *add_partition(struct gendisk *disk, int partno,
if (!dev_get_uevent_suppress(ddev))
kobject_uevent(&pdev->kobj, KOBJ_ADD);

+ kref_init(&p->ref);
return p;

out_free_info:
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index aae86fd..482a7fd 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -115,6 +115,7 @@ struct request {
void *elevator_private3;

struct gendisk *rq_disk;
+ struct hd_struct *part;
unsigned long start_time;
#ifdef CONFIG_BLK_CGROUP
unsigned long long start_time_ns;
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 7a7b9c1..2ba2792 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -116,6 +116,7 @@ struct hd_struct {
struct disk_stats dkstats;
#endif
struct rcu_head rcu_head;
+ struct kref ref;
};

#define GENHD_FL_REMOVABLE 1
@@ -583,6 +584,7 @@ extern struct hd_struct * __must_check add_partition(struct gendisk *disk,
sector_t len, int flags,
struct partition_meta_info
*info);
+extern void __delete_partition(struct kref *ref);
extern void delete_partition(struct gendisk *, int);
extern void printk_all_partitions(void);

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 6cc38fc..90b9e44 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -23,6 +23,7 @@ struct kref {

void kref_init(struct kref *kref);
void kref_get(struct kref *kref);
+int kref_test_and_get(struct kref *kref);
int kref_put(struct kref *kref, void (*release) (struct kref *kref));

#endif /* _KREF_H_ */
diff --git a/lib/kref.c b/lib/kref.c
index d3d227a..5f663b9 100644
--- a/lib/kref.c
+++ b/lib/kref.c
@@ -37,6 +37,22 @@ void kref_get(struct kref *kref)
}

/**
+ * kref_test_and_get - increment refcount for object only if refcount is not
+ * zero.
+ * @kref: object.
+ *
+ * Return non-zero if the refcount was incremented, 0 otherwise
+ */
+int kref_test_and_get(struct kref *kref)
+{
+ int ret;
+ smp_mb__before_atomic_inc();
+ ret = atomic_inc_not_zero(&kref->refcount);
+ smp_mb__after_atomic_inc();
+ return ret;
+}
+
+/**
* kref_put - decrement refcount for object.
* @kref: object.
* @release: pointer to the function that will clean up the object when the

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