netdev - Re: [PATCH net-next v2 0/3] ethtool: allow nesting of begin() and complete() callbacks

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Message-ID: <20200106084156.GA10460@netronome.com>
Date:   Mon, 6 Jan 2020 09:41:57 +0100
From:   Simon Horman <simon.horman@...ronome.com>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Maya Erez <merez@...eaurora.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless@...r.kernel.org, wil6210@....qualcomm.com,
        Francois Romieu <romieu@...zoreil.com>,
        linux-kernel@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [PATCH net-next v2 0/3] ethtool: allow nesting of begin() and
 complete() callbacks

On Mon, Jan 06, 2020 at 07:39:26AM +0100, Michal Kubecek wrote:
> The ethtool ioctl interface used to guarantee that ethtool_ops callbacks
> were always called in a block between calls to ->begin() and ->complete()
> (if these are defined) and that this whole block was executed with RTNL
> lock held:
> 
> 	rtnl_lock();
> 	ops->begin();
> 	/* other ethtool_ops calls */
> 	ops->complete();
> 	rtnl_unlock();
> 
> This prevented any nesting or crossing of the begin-complete blocks.
> However, this is no longer guaranteed even for ioctl interface as at least
> ethtool_phys_id() releases RTNL lock while waiting for a timer. With the
> introduction of netlink ethtool interface, the begin-complete pairs are
> naturally nested e.g. when a request triggers a netlink notification.
> 
> Fortunately, only minority of networking drivers implements begin() and
> complete() callbacks and most of those that do, fall into three groups:
> 
>   - wrappers for pm_runtime_get_sync() and pm_runtime_put()
>   - wrappers for clk_prepare_enable() and clk_disable_unprepare()
>   - begin() checks netif_running() (fails if false), no complete()
> 
> First two have their own refcounting, third is safe w.r.t. nesting of the
> blocks.
> 
> Only three in-tree networking drivers need an update to deal with nesting
> of begin() and complete() calls: via-velocity and epic100 perform resume
> and suspend on their own and wil6210 completely serializes the calls using
> its own mutex (which would lead to a deadlock if a request request
> triggered a netlink notification). The series addresses these problems.
> 
> changes between v1 and v2:
>   - fix inverted condition in epic100 ethtool_begin() (thanks to Andrew
>     Lunn)

Reviewed-by: Simon Horman <simon.horman@...ronome.com>