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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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