netdev - Re: [PATCH net-next] net: sched: don't release block->lock when dumping chains

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Message-ID: <CAM_iQpVjfcB52RdyND5b_AejmbFKaArdZjT5Y7QDFYuC=thCsQ@mail.gmail.com>
Date:   Fri, 1 Mar 2019 16:08:03 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] net: sched: don't release block->lock when
 dumping chains

On Thu, Feb 28, 2019 at 6:53 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> On Wed 27 Feb 2019 at 23:03, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Tue, Feb 26, 2019 at 8:10 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >>
> >>
> >> On Tue 26 Feb 2019 at 00:15, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >> > On Mon, Feb 25, 2019 at 7:45 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >> >>
> >> >> Function tc_dump_chain() obtains and releases block->lock on each iteration
> >> >> of its inner loop that dumps all chains on block. Outputting chain template
> >> >> info is fast operation so locking/unlocking mutex multiple times is an
> >> >> overhead when lock is highly contested. Modify tc_dump_chain() to only
> >> >> obtain block->lock once and dump all chains without releasing it.
> >> >>
> >> >> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
> >> >> Suggested-by: Cong Wang <xiyou.wangcong@...il.com>
> >> >
> >> > Thanks for the followup!
> >> >
> >> > Isn't it similar for __tcf_get_next_proto() in tcf_chain_dump()?
> >> > And for tc_dump_tfilter()?
> >>
> >> Not really. These two dump all tp filters and not just a template, which
> >> is O(n) on number of filters and can be slow because it calls hw offload
> >> API for each of them. Our typical use-case involves periodic filter dump
> >> (to update stats) while multiple concurrent user-space threads are
> >> updating filters, so it is important for them to be able to execute in
> >> parallel.
> >
> > Hmm, but if these are read-only, you probably don't even need a
> > mutex, you can just use RCU read lock to protect list iteration
> > and you still can grab the refcnt in the same way.
>
> That is how it worked in my initial implementation. However, it doesn't
> work with hw offloads because driver callbacks can sleep.

Hmm? You drop RCU read lock after grabbing the refcnt,
right? If so what's the problem with sleeping?