Synchronization of per-partition ecc_stats

[Daniel Ehrenberg]

Hi MTD group,

I'm working on a patch to add some more counters to MTD for monitoring
write and erase errors. I was looking at the way ECC stats are
recorded, in particular how each partition gets its ecc_stats.failures
counter.

Globally, for NAND, this counter is synchronized by
nand_get_device/nand_release_device. For individual partitions, it
looks like ecc_stats.failures is calculated by looking at how the
global stat changes across an operation sent down to the lower layer.
Here's the main place it happens:

static int part_read(struct mtd_info *mtd, loff_t from, size_t len,
                size_t *retlen, u_char *buf)
{
        struct mtd_part *part = PART(mtd);
        struct mtd_ecc_stats stats;
        int res;

        stats = part->master->ecc_stats;
        res = part->master->_read(part->master, from + part->offset, len,
                                  retlen, buf);
        if (unlikely(mtd_is_eccerr(res)))
                mtd->ecc_stats.failed +=
                        part->master->ecc_stats.failed - stats.failed;
        else
                mtd->ecc_stats.corrected +=
                        part->master->ecc_stats.corrected - stats.corrected;
        return res;
}

I'm having trouble seeing how the MTD code prevents a race: it seems
to me that if a read to either the same or different partition occurs
and also changes master->ecc_stats, the stats could be double counted.
Is there any synchronization mechanism that I'm missing?

Thanks,
Daniel Ehrenberg