On Thu, Dec 23, 2010 at 04:10:04PM +0100, Jerome Marchand wrote:
quoted text
> 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?

It might happen that kref_get(part) is called after kref_put() has been
called and reference has reached 0 and and delete_parition() call has
been scheduled as soon as rcu grace period is over. So if you do
kref_get() after that, it is not going to help.

quoted text
> 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.

Conceptually it kind of makes sense to me. So if even if we get the
pointer to partition under rcu_read_lock(), we will not account the IO
to partition if it is going away.

quoted text
> 
> > 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))
> +			/*

Do we have to check this part->partno both while taking and releasing
reference. Can't we take one extra reference for disk->part0, at
alloc_disk_node() time so that it is never freed and only freed when
disk is going away and gendisk is being freed.

That way, you don't have to differentiate between disk->part0 and rest
of the partitions while taking or dropping references.

Thanks
Vivek

quoted text
> +			 * 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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