netdev - Re: [PATCH] Revert "batman-adv: prefer kfree_rcu() over call

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Message-ID: <Zmm2uTHTge-i3eCM@sellars>
Date: Wed, 12 Jun 2024 16:54:49 +0200
From: Linus Lüssing <linus.luessing@...3.blue>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: b.a.t.m.a.n@...ts.open-mesh.org, Dmitry Antipov <dmantipov@...dex.ru>,
	netdev@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu()
 with free-only callbacks"

On Wed, Jun 12, 2024 at 04:39:15PM +0200, Linus Lüssing wrote:
> On Wed, Jun 12, 2024 at 07:06:04AM -0700, Paul E. McKenney wrote:
> > Let me make sure that I understand...
> > 
> > You need rcu_barrier() to wait for any memory passed to kfree_rcu()
> > to actually be freed?  If so, please explain why you need this, as
> > in what bad thing happens if the actual kfree() happens later.
> > 
> > (I could imagine something involving OOM avoidance, but I need to
> > hear your code's needs rather than my imaginations.)
> > 
> > 							Thanx, Paul
> [...]
> As far as I understand before calling kmem_cache_destroy()
> we need to ensure that all previously allocated objects on this
> kmem-cache were free'd. At least we get this kernel splat
> (from Slub?) otherwise. I'm not quite sure if any other bad things
> other than this noise in dmesg would occur though. Other than a
> [...]

I guess, without knowing the details of RCU and Slub, that at
least nothing super serious, like a segfault, can happen when
the remaining execution is just a kfree(), which won't need
access to batman-adv internal functions anymore.