netdev - Re: [PATCH net-next v3 7/7] selftests: net: fdb_notify: Add a test for FDB notifications

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Message-ID: <875xorjq37.fsf@nvidia.com>
Date: Wed, 13 Nov 2024 12:46:10 +0100
From: Petr Machata <petrm@...dia.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: Petr Machata <petrm@...dia.com>, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	<netdev@...r.kernel.org>, Simon Horman <horms@...nel.org>, Ido Schimmel
	<idosch@...dia.com>, Amit Cohen <amcohen@...dia.com>, Vladimir Oltean
	<vladimir.oltean@....com>, Andy Roulin <aroulin@...dia.com>,
	<mlxsw@...dia.com>, Shuah Khan <skhan@...uxfoundation.org>, Shuah Khan
	<shuah@...nel.org>, Benjamin Poirier <bpoirier@...dia.com>, Hangbin Liu
	<liuhangbin@...il.com>, <linux-kselftest@...r.kernel.org>, Jiri Pirko
	<jiri@...nulli.us>
Subject: Re: [PATCH net-next v3 7/7] selftests: net: fdb_notify: Add a test
 for FDB notifications


Jakub Kicinski <kuba@...nel.org> writes:

> On Mon, 11 Nov 2024 18:09:01 +0100 Petr Machata wrote:
>> Check that only one notification is produced for various FDB edit
>> operations.
>> 
>> Regarding the ip_link_add() and ip_link_master() helpers. This pattern of
>> action plus corresponding defer is bound to come up often, and a dedicated
>> vocabulary to capture it will be handy. tunnel_create() and vlan_create()
>> from forwarding/lib.sh are somewhat opaque and perhaps too kitchen-sinky,
>> so I tried to go in the opposite direction with these ones, and wrapped
>> only the bare minimum to schedule a corresponding cleanup.
>
> Looks like it fails about half of the time :(
>
> https://netdev.bots.linux.dev/flakes.html?min-flip=0&tn-needle=fdb-notify&br-cnt=200

OK, I can't reproduce this. Trying in VM, on an actual HW, debug, no
debug, no luck. But I see basically two failures:

- A "0 seen, 1 expected", which... I don't know, maybe it could just be
  a misplaced sleep. I don't see how, but it's a deterministing
  scenario, there shouldn't be anything racy here, either it emits or it
  doesn't, so some buffering issue is the only thing I can think of.

- Deadlocks. E.g. this, which looks like it deadlocked and timed out
  ("bad unlock balance detected" followed by "blocked for more than 122
  seconds" et.al.):

    https://netdev-3.bots.linux.dev/vmksft-net-dbg/results/846621/18-fdb-notify-sh/

  Like... how could this patchset even theoretically cause a deadlock?