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    Subject[PATCH] [RFC] make hd_struct->in_flight atomic to avoid diskstat corruption
    Date
    The disk statistics exported to userspace through proc and sysfs are not
    protected by locks to avoid performance overhead. Since most of the statistics
    are maintained in the per_cpu struct disk_stats, the chances of them getting
    corrupted is negligible. But the in_flight counter, that records the no of
    requests currently in progress is not per-cpu. This increases the chance of it
    getting corrupted. And corruption of this value would result in visibly
    distorted statistics such as negative in_flight. This can be avoided by making
    this field atomic.

    Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

    ---

    diff --git a/block/blk-core.c b/block/blk-core.c
    index 07ab754..c295deb 100644
    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -1013,12 +1013,15 @@ static inline void add_request(struct request_queue *q, struct request *req)
    static void part_round_stats_single(int cpu, struct hd_struct *part,
    unsigned long now)
    {
    + int in_flight;
    +
    if (now == part->stamp)
    return;

    - if (part->in_flight) {
    + in_flight = atomic_read(&part->in_flight);
    + if (in_flight) {
    __part_stat_add(cpu, part, time_in_queue,
    - part->in_flight * (now - part->stamp));
    + in_flight * (now - part->stamp));
    __part_stat_add(cpu, part, io_ticks, (now - part->stamp));
    }
    part->stamp = now;
    diff --git a/block/genhd.c b/block/genhd.c
    index a9ec910..9436991 100644
    --- a/block/genhd.c
    +++ b/block/genhd.c
    @@ -1028,7 +1028,7 @@ static int diskstats_show(struct seq_file *seqf, void *v)
    part_stat_read(hd, merges[1]),
    (unsigned long long)part_stat_read(hd, sectors[1]),
    jiffies_to_msecs(part_stat_read(hd, ticks[1])),
    - hd->in_flight,
    + atomic_read(&hd->in_flight),
    jiffies_to_msecs(part_stat_read(hd, io_ticks)),
    jiffies_to_msecs(part_stat_read(hd, time_in_queue))
    );
    diff --git a/fs/partitions/check.c b/fs/partitions/check.c
    index 99e33ef..ae1f55a 100644
    --- a/fs/partitions/check.c
    +++ b/fs/partitions/check.c
    @@ -241,7 +241,7 @@ ssize_t part_stat_show(struct device *dev,
    part_stat_read(p, merges[WRITE]),
    (unsigned long long)part_stat_read(p, sectors[WRITE]),
    jiffies_to_msecs(part_stat_read(p, ticks[WRITE])),
    - p->in_flight,
    + atomic_read(&p->in_flight),
    jiffies_to_msecs(part_stat_read(p, io_ticks)),
    jiffies_to_msecs(part_stat_read(p, time_in_queue)));
    }
    diff --git a/include/linux/genhd.h b/include/linux/genhd.h
    index 634c530..5921400 100644
    --- a/include/linux/genhd.h
    +++ b/include/linux/genhd.h
    @@ -97,7 +97,7 @@ struct hd_struct {
    int make_it_fail;
    #endif
    unsigned long stamp;
    - int in_flight;
    + atomic_t in_flight;
    #ifdef CONFIG_SMP
    struct disk_stats *dkstats;
    #else
    @@ -321,16 +321,16 @@ static inline void free_part_stats(struct hd_struct *part)

    static inline void part_inc_in_flight(struct hd_struct *part)
    {
    - part->in_flight++;
    + atomic_inc(&part->in_flight);
    if (part->partno)
    - part_to_disk(part)->part0.in_flight++;
    + atomic_inc(&part_to_disk(part)->part0.in_flight);
    }

    static inline void part_dec_in_flight(struct hd_struct *part)
    {
    - part->in_flight--;
    + atomic_dec(&part->in_flight);
    if (part->partno)
    - part_to_disk(part)->part0.in_flight--;
    + atomic_dec(&part_to_disk(part)->part0.in_flight);
    }

    /* block/blk-core.c */


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