Re: [PATCH] fixes for linked list bugs in block I/O code

[Jens Axboe <axboe@suse.de>]

On Wed, May 07 2003, Dave Peterson wrote:
> I found a couple of small linked list bugs in __make_request() in
> drivers/block/ll_rw_blk.c.  The bugs exist in both kernels
> 2.4.20 and 2.5.69.  Therefore I have made patches for both
> kernels.  The problem is that when inserting a new buffer_head
> into the linked list of buffer_head structures maintained by
> "struct request", the b_reqnext field is not initialized.
> 
> -Dave Peterson
>  dsp@llnl.gov
> 
> 
> ========== START OF 2.4.20 PATCH FOR drivers/block/ll_rw_blk.c ===========
> --- ll_rw_blk.c.old     Wed May  7 15:54:58 2003
> +++ ll_rw_blk.c.new     Wed May  7 15:58:07 2003
> @@ -973,6 +973,7 @@
>                                 insert_here = &req->queue;
>                                 break;
>                         }
> +                       bh->b_reqnext = req->bhtail->b_reqnext;

This is convoluted nonsense, bhtail->b_reqnext is NULL by definition. So
a simple

	bh->b_reqnext = NULL;

is much clearer.

>                         req->bhtail->b_reqnext = bh;
>                         req->bhtail = bh;
>                         req->nr_sectors = req->hard_nr_sectors += count;
> @@ -1061,6 +1062,7 @@
>         req->waiting = NULL;
>         req->bh = bh;
>         req->bhtail = bh;
> +       bh->b_reqnext = NULL;
>         req->rq_dev = bh->b_rdev;
>         req->start_time = jiffies;
>         req_new_io(req, 0, count);

Bart already covered why 2.5 definitely does not need it. I dunno what
to say for 2.4, to me it looks like a BUG if you pass in a buffer_head
with uninitialized b_reqnext. Why should that member be any different?

In fact, from where did you see this buffer_head coming from? Who is
submitting IO on a not properly inited bh? To me, that sounds like not a
block layer bug but an fs bug.

-- 
Jens Axboe