netdev - Re: Correct usage of dev_base

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Message-ID: <CANn89i+D+7XyYi=x2UxCrMM72GeP3u5MB0-7xruOZJGrERJ5vQ@mail.gmail.com>
Date:   Mon, 30 Nov 2020 22:00:16 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Jiri Benc <jbenc@...hat.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: Correct usage of dev_base_lock in 2020

On Mon, Nov 30, 2020 at 9:50 PM Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Mon, Nov 30, 2020 at 09:43:01PM +0100, Eric Dumazet wrote:
> > Understood, but really dev_base_lock can only be removed _after_ we
> > convert all usages to something else (mutex based, and preferably not
> > the global RTNL)
>
> Sure.
> A large part of getting rid of dev_base_lock seems to be just:
> - deleting the bogus usage from mlx4 infiniband and friends
> - converting procfs, sysfs and friends to netdev_lists_mutex
> - renaming whatever is left into something related to the RFC 2863
>   operstate.
>
> > Focusing on dev_base_lock seems a distraction really.
>
> Maybe.
> But it's going to be awkward to explain in words what the locking rules
> are, when the read side can take optionally the dev_base_lock, RCU, or
> netdev_lists_lock, and the write side can take optionally the dev_base_lock,
> RTNL, or netdev_lists_lock. Not to mention that anybody grepping for
> dev_base_lock will see the current usage and not make a lot out of it.
>
> I'm not really sure how to order this rework to be honest.

We can not have a mix of RCU /rwlock/mutex. It must be one, because of
bonding/teaming.

So all existing uses of rwlock / RCU need to be removed.

This is probably not trivial.

Perhaps you could add a temporary ndo_get_sleepable_stats64() so that
drivers can be converted one at a time.