linux-kernel - Re: [PATCH v2] misc: bcm_vk: Fix potential deadlock on &vk->ctx

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Message-Id: <2a1220fc-777c-4611-9f75-3a8e07a04850@app.fastmail.com>
Date:   Wed, 28 Jun 2023 13:56:29 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Chengfeng Ye" <dg573847474@...il.com>, scott.branden@...adcom.com,
        bcm-kernel-feedback-list@...adcom.com,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] misc: bcm_vk: Fix potential deadlock on &vk->ctx_lock

On Wed, Jun 28, 2023, at 13:29, Chengfeng Ye wrote:
> As &vk->ctx_lock is acquired by timer bcm_vk_hb_poll() under softirq
> context, other process context code should disable irq or bottom-half
> before acquire the same lock, otherwise deadlock could happen if the
> timer preempt the execution while the lock is held in process context
> on the same CPU.
>
> Possible deadlock scenario
> bcm_vk_open()
>     -> bcm_vk_get_ctx()
>     -> spin_lock(&vk->ctx_lock)
> 	<timer iterrupt>
> 	-> bcm_vk_hb_poll()
> 	-> bcm_vk_blk_drv_access()
> 	-> spin_lock_irqsave(&vk->ctx_lock, flags) (deadlock here)
>
> This flaw was found using an experimental static analysis tool we are
> developing for irq-related deadlock, which reported the following
> warning when analyzing the linux kernel 6.4-rc7 release.

The timer function does not seem to be performance critical at all,
it might be nicer to just move it into process context using
a delayed workqueue instead of a timer.

     Arnd