netdev - Re: [PATCH net-next v3 1/2] mctp: make __mctp_dev

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Message-ID: <20220222195914.6f001f66@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 22 Feb 2022 19:59:14 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jeremy Kerr <jk@...econstruct.com.au>
Cc:     Matt Johnston <matt@...econstruct.com.au>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v3 1/2] mctp: make __mctp_dev_get() take a
 refcount hold

On Tue, 22 Feb 2022 12:17:38 +0800 Matt Johnston wrote:
> Previously there was a race that could allow the mctp_dev refcount
> to hit zero:
> 
> rcu_read_lock();
> mdev = __mctp_dev_get(dev);
> // mctp_unregister() happens here, mdev->refs hits zero
> mctp_dev_hold(dev);
> rcu_read_unlock();
> 
> Now we make __mctp_dev_get() take the hold itself. It is safe to test
> against the zero refcount because __mctp_dev_get() is called holding
> rcu_read_lock and mctp_dev uses kfree_rcu().

Jeremy, did you have any specific semantics or naming scheme in mind
here? PTAL. Is it better to make __mctp_dev_get() "safe" or create
mctp_dev_get()? etc