linux-kernel - Re: [RFC][PATCH 2/7] kref: Add kref

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Date:   Wed, 16 Nov 2016 11:10:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        David Windsor <dave@...gbits.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()

On Wed, Nov 16, 2016 at 09:21:51AM +0100, Greg KH wrote:
> > What should we do about things like this (bpf_prog_put() and callbacks
> > from kernel/bpf/syscall.c):
> > 
> > 
> > static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
> > {
> >         struct user_struct *user = prog->aux->user;
> > 
> >         atomic_long_sub(prog->pages, &user->locked_vm);
> 
> Oh that's scary.  Let's just make one reference count rely on another
> one and not check things...

Its not a reference count, its a resource limit thingy. Also, isn't
stacking, or in general building an object graph, the entire point of
reference counts?

> >         free_uid(user);
> > }
> > 
> > static void __bpf_prog_put_rcu(struct rcu_head *rcu)
> > {
> >         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
> > 
> >         free_used_maps(aux);
> >         bpf_prog_uncharge_memlock(aux->prog);
> >         bpf_prog_free(aux->prog);
> > }
> > 
> > void bpf_prog_put(struct bpf_prog *prog)
> > {
> >         if (atomic_dec_and_test(&prog->aux->refcnt))
> >                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> > }
> > 
> > 
> > Not only do we want to protect prog->aux->refcnt, but I think we want
> > to protect user->locked_vm too ... I don't think it's sane for
> > user->locked_vm to be a stats_t ?
> 
> I don't think this is sane code...

I once again fail to see any problems. That code is fine.