linux-kernel - Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.

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Message-Id: <20180723205625.GZ12945@linux.vnet.ibm.com>
Date:   Mon, 23 Jul 2018 13:56:25 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     NeilBrown <neilb@...e.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] rhashtable: don't hold lock on first table
 throughout insertion.

On Mon, Jul 23, 2018 at 09:13:43AM +1000, NeilBrown wrote:
> On Sun, Jul 22 2018, Paul E. McKenney wrote:
> >
> > One issue is that the ->func pointer can legitimately be NULL while on
> > RCU's callback lists.  This happens when someone invokes kfree_rcu()
> > with the rcu_head structure at the beginning of the enclosing structure.
> > I could add an offset to avoid this, or perhaps the kmalloc() folks
> > could be persuaded Rao Shoaib's patch moving kfree_rcu() handling to
> > the slab allocators, so that RCU only ever sees function pointers in
> > the ->func field.
> >
> > Either way, this should be hidden behind an API to allow adjustments
> > to be made if needed.  Maybe something like is_after_call_rcu()?
> > This would (for example) allow debug-object checks to be used to catch
> > check-after-free bugs.
> >
> > Would something of that sort work for you?
> 
> Yes, if you could provide an is_after_call_rcu() API, that would
> perfectly suit my use-case.

After beating my head against the object-debug code a bit, I have to ask
if it would be OK for you if the is_after_call_rcu() API also takes the
function that was passed to RCU.

							Thanx, Paul