Re: please DON'T run 2.5.27 with IDE!

Bartlomiej Zolnierkiewicz
2002-07-24T12:36:57Z

On Wed, 24 Jul 2002, Marcin Dalecki wrote:

> Bartlomiej Zolnierkiewicz wrote:
>
> >>There are some nasty checks for it != NULL in the generic BIO code.
^^^^^^^^^^^^^^^^^^^^^^^
> >
> >
> > No, there are no checks there.
>
> Hello?:

Yes, hello Martin, read you own sentence ;-)

Generic BIO code is bio.c and bio.h.

> [root@localhost block]# grep \>special *.c
> elevator.c: !rq->waiting && !rq->special)
> ^^^^^^ This one is supposed to have the required barrier effect.
> ll_rw_blk.c: if (req->special || next->special)
> ll_rw_blk.c: rq->special = NULL;
> ll_rw_blk.c: rq->special = data;
> ^^^^^^^ This one is me :-).
> ll_rw_blk.c: || next->waiting || next->special)
> [root@localhost block]#

You seem to confuse block layer with BIO layer.

> >>>So look at ide.c for example.
> >>
> >>So look at drivers which call blk_start_queue() from within
> >>q->request_fn context, which is, well, causing deliberate *recursion*.
> >>
> >
> > Are you sure? If so they should first check whether queue is
> > started/stopped, if they don't it is a bug.
>
> void blk_start_queue(request_queue_t *q)
> {
> if (test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags)) {
> unsigned long flags;
>
> ================== possigle race here for qeue_flags BTW.
>
> spin_lock_irqsave(q->queue_lock, flags);
> clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
>
> if (!elv_queue_empty(q))
> q->request_fn(q);
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> If we call it from within request_fn then if this isn't recursion on the
> kernel stack then I don't know...

You really don't know.

And funny thing is I have diffirent blk_start_queue() function in my tree
(2.5.27) ? Without described above race and without possibilty of
recursion...

2.5.27:drivers/block/ll_rw_blk.c
void blk_start_queue(request_queue_t *q)
{
if (test_and_clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags)) {
unsigned long flags;

spin_lock_irqsave(q->queue_lock, flags);
if (!elv_queue_empty(q))
q->request_fn(q);
spin_unlock_irqrestore(q->queue_lock, flags);
}
}

Regards
--
Bartlomiej

> spin_unlock_irqrestore(q->queue_lock, flags);
> }
> }