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Subject: Re: [Cake] [PATCH] sch_cake: avoid possible divide by zero in
 cake_enqueue()
From:   Jonathan Morton <chromatix99@gmail.com>
In-Reply-To: <20200102092143.8971-1-wenyang@linux.alibaba.com>
Date:   Thu, 2 Jan 2020 23:58:02 +0200
Cc:     =?utf-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= <toke@toke.dk>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        cake@lists.bufferbloat.net,
        Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        "David S . Miller" <davem@davemloft.net>
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References: <20200102092143.8971-1-wenyang@linux.alibaba.com>
To:     Wen Yang <wenyang@linux.alibaba.com>
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> On 2 Jan, 2020, at 11:21 am, Wen Yang <wenyang@linux.alibaba.com> =
wrote:
>=20
> The variables 'window_interval' is u64 and do_div()
> truncates it to 32 bits, which means it can test
> non-zero and be truncated to zero for division.
> The unit of window_interval is nanoseconds,
> so its lower 32-bit is relatively easy to exceed.
> Fix this issue by using div64_u64() instead.

That might actually explain a few things.  I approve.

Honestly the *correct* fix is for the compiler to implement division in =
a way that doesn't require substituting it with function calls.  As this =
shows, it's error-prone to do this manually.

 - Jonathan Morton=