netdev - Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

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Message-ID: <20180730222657.GA197142@joelaf.mtv.corp.google.com>
Date:   Mon, 30 Jul 2018 15:26:57 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Chenbo Feng <fengc@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Alexei Starovoitov <ast@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

On Sun, Jul 29, 2018 at 06:57:06PM +0300, Alexei Starovoitov wrote:
> On Fri, Jul 27, 2018 at 10:17 PM, Daniel Colascione <dancol@...gle.com> wrote:
> > On Sat, Jul 14, 2018 at 11:18 AM, Joel Fernandes <joel@...lfernandes.org>
> > wrote:
> >>
> >> By the way just curious I was briefly going through kernel/bpf/arraymap.c.
> >> How are you protecting against load-store tearing of values of array map
> >> updates/lookups?
> >>
> >> For example, if userspace reads an array map at a particular index, while
> >> another CPU is updating it, then userspace can read partial values /
> >> half-updated values right? Since rcu_read_lock is in use, I was hoping to
> >> find something like rcu_assign_pointer there to protect readers against
> >> concurrent updates.  Thanks for any clarification.
> >
> >
> > I'm also curious about the answer to this question.
> 
> i'm not sure I understand the question.
> bpf_map_type_array is a C-like array.
> There is no locking of elements.
> If single program executing on two cpus
> and accesses the same value it will collide.
> Same goes for user space vs prog parallel access.
> bpf long_memcpy is an attempt to provide minimal
> level of automicity when values are aligned and
> size==long.

Thanks for the answer. I think you answered the question.

Regards,

- Joel