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.
