Re: [PATCH v3 2/2] make buffer_locked provide an acquire semantics

[Ard Biesheuvel]

On Mon, 1 Aug 2022 at 00:14, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sun, Jul 31, 2022 at 04:43:08PM -0400, Mikulas Patocka wrote:
> > Let's have a look at this piece of code in __bread_slow:
> >       get_bh(bh);
> >       bh->b_end_io = end_buffer_read_sync;
> >       submit_bh(REQ_OP_READ, 0, bh);
> >       wait_on_buffer(bh);
> >       if (buffer_uptodate(bh))
> >               return bh;
> > Neither wait_on_buffer nor buffer_uptodate contain a memory barrier.
> > Consequently, if someone calls sb_bread and then reads the buffer data,
> > the read of buffer data may be executed before wait_on_buffer(bh) on
> > architectures with weak memory ordering and it may return invalid data.
>
> I think we should be consistent between PageUptodate() and
> buffer_uptodate().  Here's how it's done for pages currently:
>
> static inline bool folio_test_uptodate(struct folio *folio)
>         bool ret = test_bit(PG_uptodate, folio_flags(folio, 0));
>         /*
>          * Must ensure that the data we read out of the folio is loaded
>          * _after_ we've loaded folio->flags to check the uptodate bit.
>          * We can skip the barrier if the folio is not uptodate, because
>          * we wouldn't be reading anything from it.
>          *
>          * See folio_mark_uptodate() for the other side of the story.
>          */
>         if (ret)
>                 smp_rmb();
>
>         return ret;
>
> ...
>
> static __always_inline void folio_mark_uptodate(struct folio *folio)
>         /*
>          * Memory barrier must be issued before setting the PG_uptodate bit,
>          * so that all previous stores issued in order to bring the folio
>          * uptodate are actually visible before folio_test_uptodate becomes true.
>          */
>         smp_wmb();
>         set_bit(PG_uptodate, folio_flags(folio, 0));
>
> I'm happy for these to also be changed to use acquire/release; no
> attachment to the current code.  But bufferheads & pages should have the
> same semantics, or we'll be awfully confused.

I suspect that adding acquire/release annotations at the bitops level
is not going to get us anywhere, given that for the uptodate flag, it
is the set operation that has release semantics, whereas for a lock
flag, it will be the clear operation. Reverting to the legacy barrier
instructions to try and avoid this ambiguity will likely only make
things worse.

I was cc'ed only on patch #1 of your v3, so I'm not sure where this is
headed, but I strongly +1 Matthew's point above that this should be
done at the level that defines how the bit fields should be
interpreted wrt to the contents of the data structure that they
describe/guard.