linux-kernel - Re: [bug report] RDMA/iwpm: reentrant iwpm hello message

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Message-ID: <103c061b.e87e.193f84b0840.Coremail.linma@zju.edu.cn>
Date: Tue, 24 Dec 2024 18:51:27 +0800 (GMT+08:00)
From: "Lin Ma" <linma@....edu.cn>
To: "Leon Romanovsky" <leon@...nel.org>
Cc: jgg@...pe.ca, cmeiohas@...dia.com, michaelgur@...dia.com,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [bug report] RDMA/iwpm: reentrant iwpm hello message

Hello Leon,

> 
> I'm not fully understand the lockdep here. We use down_read(), which is
> reentry safe.
> 

Really? To my knowledge, though down_read() itself will not trigger locking
errors. But below scenario will lead to deadlock, and that's why this
WARNING is raised.

   CPU0                CPU1
   ----                ----
down_read()[1]
                  down_write()[2]
down_read()[3]

If CPU1 thread not exists, the CPU0 will run smoothly (However, it will keep
looping and the PoC cannot be killed by any signal, causing Denial-of-Service).

When CPU1 calls down_write(), it will wait for [1] to be released.
However, when [3] is called, it will then wait for [2] to be released,
leading to a deadlock situation.

Please let me know if I understand this correctly or incorrectly?

Thanks,
Lin