linux-kernel - Re: [GIT PULL] bcachefs changes for 6.12-rc1

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Message-ID: <ZvdSFlaEkk7fqZVl@gondor.apana.org.au>
Date: Sat, 28 Sep 2024 08:47:18 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dave Chinner <dchinner@...hat.com>,
	Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org
Subject: Re: [GIT PULL] bcachefs changes for 6.12-rc1

On Fri, Sep 27, 2024 at 08:11:06PM -0400, Kent Overstreet wrote:
>
> > > And, when I was torture testing that code I tripped over what appeared
> > > to be an infinite loop in rht_bucket() when a rehsah is in progress,
> > > which I worked around in
> > > 
> > >   a592cdf5164d bcachefs: don't use rht_bucket() in btree_key_cache_scan()
> > 
> > You must not walk the rhashtable internal data structures by hand.
> > If you need to iterate through the whole table, use the provided
> > walking mechanism.
> 
> I was just doing rht_for_each_entry_rcu() (open coded because you don't
> provide a safe version).

I'm talking about the commit a592cd above.  You were doing this:

	for (i = 0; i < tbl->size; i++)
		rht_for_each_entry_rcu(ck, pos, tbl, i, hash) {

This is absolutely not allowed.  The rhashtable must only be accessed
through the published API and not iterated over directly.  This is
guaranteed to break during a resize/rehash.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt