Re: [PATCH] Add a read memory barrier to wait_on_buffer

[Mikulas Patocka <mpatocka@redhat.com>]

On Sun, 31 Jul 2022, Ard Biesheuvel wrote:

> This has little to do with speculation, so better to drop this S bomb
> from your commit message. This is about concurrency and weak memory
> ordering.

Yes.

> This doesn't seem like a very robust fix to me, tbh - I suppose this
> makes the symptom you encountered go away, but the underlying issue
> remains afaict.
> 
> Given that the lock and uptodate fields etc are just bits in a
> bitfield, wouldn't it be better to use cmpxchg() with acquire/release
> semantics (as appropriate) to manage these bits?

The kernel already uses clear_bit_unlock, test_and_set_bit_lock and 
wait_on_bit_lock_io to manage the BH_Lock bit - and they have 
acquire/release semantics.

The only problem is that test_bit doesn't provide any memory barriers. 
Should we add the barrier to buffer_locked() instead of wait_on_buffer()? 
Perhaps it would fix more bugs - in reiserfs, there's this piece of code:

                if (buffer_locked(bh)) {
                        spin_unlock(lock);
                        wait_on_buffer(bh);
                        spin_lock(lock);
                }
                if (!buffer_uptodate(bh)) {
                        ret = -EIO;
                }

or this:
                if (buffer_locked(bh)) {
                        int depth;
                        PROC_INFO_INC(sb, scan_bitmap.wait);
                        depth = reiserfs_write_unlock_nested(sb);
                        __wait_on_buffer(bh);
                        reiserfs_write_lock_nested(sb, depth);
                }
                BUG_ON(!buffer_uptodate(bh));
                BUG_ON(atomic_read(&bh->b_count) == 0);

That assumes that buffer_locked provides a barrier.

Mikulas