Re: doubt in chunk_aligned_read()

[Anuj Goel <talk2anuj@xxxxxxxxx>]

Thanks for the hint !!

I have few more doubts in the function,

static int chunk_aligned_read(mddev_t *mddev, struct bio * raid_bio)

if all the preliminary checks pass, we increment active_aligned_reads
and call generic_make_request() which will add this bio to the
"current->bio_list".
This being an asynchronous read, chunk_aligned_read returns and the
result of the read will be notified by the registered callback
function "raid5_align_endio".
Based on this understanding, I have some questions below:

1. How and when is the read from the disk initiated ?

2. The bio passed to raid5_align_endio() is the one which was cloned
in chunk_aligned read(). The below statement in raid5_align_endio()
extracts the original bio :
struct bio* raid_bi  = bi->bi_private;
My doubt is in the assignment,
rdev = (void*)raid_bi->bi_next;
raid_bi is the original bio and its bi_next field should have been
pointing to the next bio in the request queue "current->bio_list"  (if
active_aligned_reads is > 0 ). How can this point to a rdev structure
? Or better put,  where do we make the bio->bi_next point to a rdev
struct ?

3. after this, we have,
    i) mddev = rdev->mddev;
    ii) conf = mddev->private;
    iii) rdev_dec_pending(rdev, conf->mddev);
In the call in line iii) we could have just sent the mddev retrieved
in line i). I am curious if there is a specific reason for reading it
again from the raid configuration struct "conf" ? Could there be a
case when mddev in line i) can be different from the conf->mddev in
line iii)  ??

On Wed, Apr 4, 2012 at 9:38 PM, NeilBrown <neilb@xxxxxxx> wrote:
> On Wed, 4 Apr 2012 21:25:53 -0400 Anuj Goel <talk2anuj@xxxxxxxxx> wrote:
>
>> HI Guys,
>>
>> Why is the below call needed in the function  chunk_aligned_read()
>>
>>       align_bi = bio_clone_mddev(raid_bio, GFP_NOIO, mddev);
>>
>> Why do we need to create a clone of the bio passed to
>> chunk_aligned_read(mddev_t *mddev, struct bio * raid_bio) ??
>
> Think about what happens if the read fails.
>
> NeilBrown
>
>
>>
>> --
>> Best Regards,
>> Anuj Goel
>> --
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>



-- 
Best Regards,
Anuj Goel
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