[Drbd-dev] Re: [DRBD-cvs] r1743 - in branches/drbd-0.7: . drbd

[Philipp Reisner]

Am Mittwoch, 2. Februar 2005 11:13 schrieb Lars Marowsky-Bree:
> On 2005-02-01T18:36:43, Philipp Reisner <philipp.reisner at linbit.com> wrote:
> > I did not look at the comments, I read the code. And what I read there
> > was: a BIO has after bio_init() a refcount (bi_cnt) of 1. It is freed
> > when the refcount drops to zero.
> >
> > bio_alloc();
> > do something with it
>
> "submit_bio()" is slightly different from just "something".

in which regard ?  We should not free it while we do something
with it.. that is true for something and for submit_bio() :)

> > bio_put();
> >
> > is right.
> >
> > What you did is:
> >
> > bio_alloc();
> > bio_get();   // increase to 2
> > bio_put();   // decreat to 1
> >
> > and let it live forever...
>
> The cleanup at the end of the bio processing however should drop the bio
> reference count too. Which is why the comment in bio.h says what it is.
> Either the comment is wrong (and Jens's very same fix for the md code,
> too), or you have introduced a race condition into the actlog.

Why do you not simply read the code ? 

Right in case you return from the function that alocated the bio, then
it is a good idea to use the reference counting to make the completion
handler to free it. This would be something like this:

STATIC int _drbd_md_async_page_io(...)
{
 struct bio *bio = bio_alloc(GFP_KERNEL, 1);

 bio->bi_end_io = drbd_md_io_complete_a;
 submit_bio(rw, bio);

 return;
}

You see, there is no single bio_get() or bio_put() ... What refcount
will the bio have ? Right it will have one (1), so the bio lives on...

After some while, when the disk was so nice to write our request the
bio completion function would be called:

int drbd_md_io_complete_a(struct bio *bio, unsigned int bytes_done, int error)
{
 if (bio->bi_size)
  return 1;

 put_bio(bio);
 return 0;
}

And here we would decrease the BIOs refcount to 0 and free it.

BUT the function we talk about has SYNCHRONOUS semantics! Otherwise
it would have never worked with a BIO on the stack... Read carefull:

STATIC int _drbd_md_sync_page_io(drbd_dev *mdev, struct page *page, 
     sector_t sector, int rw, int size)
{
 struct bio *bio = bio_alloc(GFP_KERNEL, 1);

 bio->bi_end_io = drbd_md_io_complete;

 wait_for_completion(&event);

 bio_put(bio);
 return ok;
}

and the corresponding io-completion function:

int drbd_md_io_complete(struct bio *bio, unsigned int bytes_done, int error)
{
 if (bio->bi_size)
  return 1;

 complete((struct completion*)bio->bi_private);
 return 0;
}

So our _drbd_md_sync_page_io() sleeps in wait_for_completion() until the
whole action is carried out, and then ... in the same scope where I allocated
the bio I free it by calling bio_put() --- 
I could do it on the stack -- But I should use bio_init() the... 

Jens: 
      Please tell Lars that I am right on how the refcounting works.

      And, BTW, is there any reason to not have a BIO on the stack ?
      Besides people return from the function before the completion callback
      was called ?

      In other words: Might something below me increase the refcount of the
      BIO and keep the reference even after my IO completion function was
      called ?

-Philipp