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

[Philipp Reisner]

Am Dienstag, 1. Februar 2005 17:48 schrieb Lars Marowsky-Bree:
> On 2005-01-31T12:19:21, svn at svn.drbd.org wrote:
> > Modified:
> >    branches/drbd-0.7/ChangeLog
> >    branches/drbd-0.7/drbd.spec.in
> >    branches/drbd-0.7/drbd/drbd_actlog.c
> >    branches/drbd-0.7/drbd/drbd_compat_wrappers.h
> > Log:
> > No longer leak of BIOs.
>
> Good goal, but ...
>
> > Modified: branches/drbd-0.7/drbd/drbd_actlog.c
> > ===================================================================
> > --- branches/drbd-0.7/drbd/drbd_actlog.c 2005-01-31 09:20:39 UTC (rev
> > 1742) +++ branches/drbd-0.7/drbd/drbd_actlog.c 2005-01-31 11:19:19 UTC
> > (rev 1743) @@ -68,8 +68,6 @@
> >   struct completion event;
> >   int ok;
> >
> > - bio_get(bio);
> > -
> >   bio->bi_bdev = mdev->md_bdev;
> >   bio->bi_sector = sector;
> >   bio_add_page(bio, page, size, 0);
>
> I think this is a bug. If you look at the comments in
> linux/include/linux/bio.h, you'll find that this is just how the bio is
> supposed to be used:
>
> /*
>  * get a reference to a bio, so it won't disappear. the intended use is
>  * something like:
>  *
>  * bio_get(bio);
>  * submit_bio(rw, bio);
>  * if (bio->bi_flags ...)
>  *      do_something
>  * bio_put(bio);
>  *
>  * without the bio_get(), it could potentially complete I/O before
> submit_bio * returns. and then bio would be freed memory when if
> (bio->bi_flags ...) * runs
>  */
> #define bio_get(bio)    atomic_inc(&(bio)->bi_cnt)
>
> The fix I did was not the right way (removing the bio embedding was much
> better), but it also wasn't completely wrong.
>

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
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...

-Philipp
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