linux-kernel - Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache

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Message-ID: <62757652-8874-45d7-afec-734edeb03831@paulmck-laptop>
Date: Thu, 13 Jun 2024 07:53:24 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
	Julia Lawall <Julia.Lawall@...ia.fr>, linux-block@...r.kernel.org,
	kernel-janitors@...r.kernel.org, bridge@...ts.linux.dev,
	linux-trace-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	kvm@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Nicholas Piggin <npiggin@...il.com>, netdev@...r.kernel.org,
	wireguard@...ts.zx2c4.com, linux-kernel@...r.kernel.org,
	ecryptfs@...r.kernel.org, Neil Brown <neilb@...e.de>,
	Olga Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>,
	Tom Talpey <tom@...pey.com>, linux-nfs@...r.kernel.org,
	linux-can@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On Thu, Jun 13, 2024 at 07:17:38AM -0700, Jakub Kicinski wrote:
> On Wed, 12 Jun 2024 20:38:02 -0700 Paul E. McKenney wrote:
> > o	Make rcu_barrier() wait for kfree_rcu() objects.  (This is
> > 	surprisingly complex and will wait unnecessarily in some cases.
> > 	However, it does preserve current code.)
> 
> Not sure how much mental capacity for API variations we expect from
> people using caches, but I feel like this would score the highest
> on Rusty's API scale. I'd even venture an opinion that it's less
> confusing to require cache users to have their own (trivial) callbacks
> than add API variants we can't error check even at runtime...

Fair point, though please see Jason's emails.

And the underlying within-RCU mechanism is the same either way, so that
API decision can be deferred for some time.

But the within-slab mechanism does have the advantage of also possibly
simplifying reference-counting and the potential upcoming hazard pointers.
On the other hand, I currently have no idea what level of violence this
change would make to the slab subsystem.

							Thanx, Paul