Re: [BUG] bcachefs (early?) bucket allocator raciness

[Brian Foster <bfoster@redhat.com>]

On Thu, Oct 19, 2023 at 11:52:11AM -0400, Kent Overstreet wrote:
> On Thu, Oct 19, 2023 at 09:37:24AM -0400, Brian Foster wrote:
> > Hi Kent, All,
> > 
> > I recently observed a data corruption problem that is related to the
> > recently discovered issue of mounted fs' running with the early bucket
> > allocator instead of the freelist allocator. The immediate failure is
> > generic/113 producing various splats, the most common of which is a
> > duplicate backpointer issue. generic/113 is primarily an aio/dio stress
> > test.
> > 
> > I eventually tracked this down to an actual duplicate bucket allocation
> > in the early bucket allocator code. The race generally looks as follows:
> > 
> > - Task 1 lands in bch2_bucket_alloc_early(), selects key A from the
> >   alloc btree, and then schedules (perhaps due to freelist_lock).
> > 
> > - Task 2 runs through the same alloc path and selects the same key K,
> >   but proceeds to open the associated bucket, alloc/write to it,
> >   complete the I/O and release the bucket (removing it from the hash).
> > 
> > - Task 1 continues with alloc key K. bch2_bucket_is_open() returns false
> >   because the previously opened bucket has been removed from the hash
> >   list. Therefore task 1 opens a new bucket for what is now no longer free
> >   space and uses it for the its associated write operation.
> 
> This shouldn't be possible because task 1 is holding the alloc key
> locked, and task 2 has to update that same alloc key before releasing
> the open bucket.
> 
> Except perhaps not - perhaps this is a key cache coherency issue?
> 
> We're not using a BTREE_ITER_CACHED iterator, because we're scanning and
> we can't scan with key cache iterators. It's still supposed to be
> coherent with the key cache; bch2_btree_iter_peek_slot() ->
> btree_trans_peek_key_cache() checks if a key exists in the key cache and
> returns that key instead of the key in the btree if it exists.
> 
> But it doesn't return with that slot locked in the key cache locked if
> the key didn't exist in the key cache. Oops.
> 

Ah, interesting. I wasn't aware of the lower level locking involved
here. This sounds like a plausible theory wrt key cache, but I'll have
to dig more into it to grok the locking. Thanks for the additional
context.

Brian

> So we're going to need to keep an eye out for this issue occuring
> elsewhere, and maybe come up with a real fix in the btree iterator code:
> looking up a key in a cached btree without BTREE_ITER_CACHED _does_
> return the correct key at that particular point in time, but it does
> _not_ necessarily keep it locked for the duration of the transaction.
> 
> For now, we can fix this locally in bch2_bucket_alloc_early() with a
> second BTREE_ITER_CACHED iterator - run some tests with freespace
> initialization disabled, confirm that that's the issue, then go from
> there.
>