Re: splice: infinite busy loop lockup bug

Ming Lei
2020-08-09T02:31:49Z

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