linux-kernel - Re: [PATCH] generic dynamic per cpu refcounting

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130125191139.GA19247@redhat.com>
Date:	Fri, 25 Jan 2013 20:11:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	tj@...nel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] generic dynamic per cpu refcounting

On 01/25, Oleg Nesterov wrote:
>
> > +int percpu_ref_kill(struct percpu_ref *ref)
> > +{
> > ...
> > +	if (status == PCPU_REF_PTR) {
> > +		unsigned count = 0, cpu;
> > +
> > +		synchronize_rcu();
> > +
> > +		for_each_possible_cpu(cpu)
> > +			count += *per_cpu_ptr((unsigned __percpu *) pcpu_count, cpu);
> > +
> > +		pr_debug("global %lli pcpu %i",
> > +			 atomic64_read(&ref->count) & PCPU_COUNT_MASK,
> > +			 (int) count);
> > +
> > +		atomic64_add((int) count, &ref->count);
> > +		smp_wmb();
> > +		/* Between setting global count and setting PCPU_REF_DEAD */
> > +		ref->pcpu_count = PCPU_REF_DEAD;
>
> The coment explains what the code does, but not why ;)
>
> I guess this is for percpu_ref_put(), and this wmb() pairs with implicit
> mb() implied by atomic64_dec_return().

Hmm. Most probably I missed something, but it seems we need another
synchronize_rcu() _after_ we set PCPU_REF_DEAD.

To simplify, suppose that percpu_ref_put() is never called directly but
we have

	void put_and_dsetroy(...)
	{
		if (percpu_ref_put(...))
			destroy(...);
	}

Suppose that ref->count == 2 after atomic64_add() above. IOW, we have
a "master" reference for _kill() and someone else did _get.

So the caller does

	percpu_ref_kill();
	put_and_dsetroy();

And this can race with another holder which drops the last reference,
its put_and_dsetroy() can see PCPU_REF_DYING and return false.

Or I misunderstood the code/interface?

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/