Re: splice: infinite busy loop lockup bug

[Ming Lei]

On Fri, Aug 07, 2020 at 03:11:48PM +0100, Matthew Wilcox wrote:
> On Fri, Aug 07, 2020 at 09:41:14PM +0800, Ming Lei wrote:
> > On Fri, Aug 07, 2020 at 01:38:54PM +0100, Al Viro wrote:
> > > FWIW, my preference would be to have for_each_bvec() advance past zero-length
> > > segments; I'll need to go through its uses elsewhere in the tree first, though
> > > (after I grab some sleep),
> > 
> > Usually block layer doesn't allow/support zero bvec, however we can make
> > for_each_bvec() to support it only.
> > 
> > Tetsuo, can you try the following patch?
> > 
> > diff --git a/include/linux/bvec.h b/include/linux/bvec.h
> > index ac0c7299d5b8..b03c793dd28d 100644
> > --- a/include/linux/bvec.h
> > +++ b/include/linux/bvec.h
> > @@ -117,11 +117,19 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
> >  	return true;
> >  }
> >  
> > +static inline void bvec_iter_skip_zero_vec(const struct bio_vec *bv,
> > +		struct bvec_iter *iter)
> > +{
> > +	iter->bi_idx++;
> > +	iter->bi_bvec_done = 0;
> > +}
> > +
> >  #define for_each_bvec(bvl, bio_vec, iter, start)			\
> >  	for (iter = (start);						\
> >  	     (iter).bi_size &&						\
> > -		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
> > -	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
> > +		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);		\
> > +	  (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len) : \
> > +			bvec_iter_skip_zero_vec((bio_vec), &(iter)))
> 
> Uhm, bvec_iter_advance() already skips over zero length bio_vecs.
> 
>         while (bytes && bytes >= bv[idx].bv_len) {
>                 bytes -= bv[idx].bv_len;
>                 idx++;
>         }

The issue is that zero (bvl).bv_len passed to bvec_iter_advance(), so
the iterator can't move on.

And I tried to avoid change to bvec_iter_advance() since this exact
issue only exists on for_each_bvec, and block layer won't support/allow
zero-length bvec.

> 
> The problem is when the _first_ bio_vec is zero length.

It can be any zero-length bvec during the iterating. 

> Maybe something more
> like this (which doesn't even compile, but hopefully makes my point):
> 
> @@ -86,12 +86,24 @@ struct bvec_iter_all {
>         (mp_bvec_iter_page((bvec), (iter)) +                    \
>          mp_bvec_iter_page_idx((bvec), (iter)))
>  
> -#define bvec_iter_bvec(bvec, iter)                             \
> -((struct bio_vec) {                                            \
> -       .bv_page        = bvec_iter_page((bvec), (iter)),       \
> -       .bv_len         = bvec_iter_len((bvec), (iter)),        \
> -       .bv_offset      = bvec_iter_offset((bvec), (iter)),     \
> -})
> +static inline bool bvec_iter_bvec(struct bio_vec *bv, struct bio_vec *bvec,
> +               struct bvec_iter *iter)
> +{
> +       unsigned int idx = iter->bi_idx;
> +
> +       if (!iter->bi_size)
> +               return false;
> +
> +       while (!bv[idx].bv_len)
> +               idx++;
> +       iter->bi_idx = idx;
> +
> +       bv->bv_page = bvec_iter_page(bvec, *iter);
> +       bv->bv_len = bvec_iter_len(bvec, *iter);
> +       bv->bv_offset = bvec_iter_offset(bvec, *iter);
> +
> +       return true;
> +}
>  
>  static inline bool bvec_iter_advance(const struct bio_vec *bv,
>                 struct bvec_iter *iter, unsigned bytes)
> @@ -119,8 +131,7 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
>  
>  #define for_each_bvec(bvl, bio_vec, iter, start)                       \
>         for (iter = (start);                                            \
> -            (iter).bi_size &&                                          \
> -               ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \
> +            bvec_iter_bvec(&(bvl), (bio_vec), &(iter));                \
>              bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
>  
>  /* for iterating one bio from start to end */
> 
> (I find the whole bvec handling a mess of confusing macros and would
> welcome more of it being inline functions, in general).

The above change may bring more code duplication. Meantime, it can't
work because (bvl).bv_len isn't taken into account into bvec_iter_bvec(),
then how can the iterator advance?


Thanks,
Ming