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Date:   Thu, 3 Sep 2020 17:44:14 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>, Lorenz Bauer <lmb@...udflare.com>,
        Martin KaFai Lau <kafai@...com>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf 0/2] bpf: do not use bucket_lock for hashmap iterator

On Wed, Sep 2, 2020 at 4:54 PM Yonghong Song <yhs@...com> wrote:
>
> Currently, the bpf hashmap iterator takes a bucket_lock, a spin_lock,
> before visiting each element in the bucket. This will cause a deadlock
> if a map update/delete operates on an element with the same
> bucket id of the visited map.
>
> To avoid the deadlock, let us just use rcu_read_lock instead of
> bucket_lock. This may result in visiting stale elements, missing some elements,
> or repeating some elements, if concurrent map delete/update happens for the
> same map. I think using rcu_read_lock is a reasonable compromise.
> For users caring stale/missing/repeating element issues, bpf map batch
> access syscall interface can be used.
>
> Note that another approach is during bpf_iter link stage, we check
> whether the iter program might be able to do update/delete to the visited
> map. If it is, reject the link_create. Verifier needs to record whether
> an update/delete operation happens for each map for this approach.
> I just feel this checking is too specialized, hence still prefer
> rcu_read_lock approach.
>
> Patch #1 has the kernel implementation and Patch #2 added a selftest
> which can trigger deadlock without Patch #1.

Applied. Thanks

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