netdev - Re: [PATCH 0/9] net: bridge: reduce multicast checks in fast path

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Message-ID: <bfb11627-64d5-42a0-911e-8be99e222396@blackwall.org>
Date: Fri, 29 Aug 2025 19:23:24 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Jakub Kicinski <kuba@...nel.org>, Linus Lüssing
 <linus.luessing@...3.blue>
Cc: bridge@...ts.linux.dev, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Ido Schimmel <idosch@...dia.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, Simon Horman <horms@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
 "David S . Miller" <davem@...emloft.net>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, Stanislav Fomichev <sdf@...ichev.me>,
 Xiao Liang <shaw.leon@...il.com>
Subject: Re: [PATCH 0/9] net: bridge: reduce multicast checks in fast path

On 8/29/25 18:47, Jakub Kicinski wrote:
> On Fri, 29 Aug 2025 10:53:41 +0200 Linus Lüssing wrote:
>> This patchset introduces new state variables to combine and reduce the
>> number of checks we would otherwise perform on every multicast packet
>> in fast/data path.
>>    
>> The second reason for introducing these new, internal multicast active
>> variables is to later propagate a safety mechanism which was introduced
>> in b00589af3b04 ("bridge: disable snooping if there is no querier") to
>> switchdev/DSA, too. That is to notify switchdev/DSA if multicast
>> snooping can safely be applied without potential packet loss.
> 
> Please leave the git-generated diff stat in the cover letter.
> Please include tree designation in the subject, per:
> https://www.kernel.org/doc/html/next/process/maintainer-netdev.html
> 
> I'll leave the real review to the experts but this series appears
> to make kselftests unhappy:
> 

just fyi my email wasn't working for 2 days and unfortunately I missed this set
I took a look now on patchwork, I do have comments but it's difficult to reply as
I don't have the emails and have to do it manually to each, so I'd rather wait
for v2.

a few notes for v2:
- please use READ/WRTE_ONCE() for variables that are used without locking
- please make locking symmetric, I saw that br_multicast_open() expects the lock to be already held, while
   __br_multicast_stop() takes it itself
- target net-next
- is the mcast lock really necessary, would atomic ops do for this tracking?
- can you provide the full view somewhere, how would this tracking be used? I fear
   there might still be races.
- please add more details exactly what we save on the fast-path, I know but it'd be nice
   to have it in the commit message as well, all commits just say "reduce checks, save cycles"
   but there are no details what we save

I will try to give more detailed comments in v2.

Thank you,
  Nik