From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net] ipv6/mcast: update mc_qrv when join new group
Date: Mon, 29 Oct 2018 20:23:56 -0700 (PDT)
Message-ID: <20181029.202356.802822894497287155.davem@davemloft.net>
References: <1540521054-17825-1-git-send-email-liuhangbin@gmail.com>
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From: Hangbin Liu <liuhangbin@gmail.com>
Date: Fri, 26 Oct 2018 10:30:54 +0800

> Currently we only set mc_qrv to sysctl_mld_qrv when interface up. If we
> change sysctl_mld_qrv after interface up, it will has no effect.
> 
> Fix it by assigning latest sysctl_mld_qrv to idev mc_qrv when join new group.
> 
> Reported-by: Ying Xu <yinxu@redhat.com>
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>

Why isn't mld_update_qrv() taking care of this?

At a minimum, you must make your change take MLD_QRV_DEFAULT etc.
into account like mld_update_qrv() does.