Re: [PATCH 1/2] PCI: mediatek-gen3: Stop acquiring spinlocks in {suspend,resume}_noirq

From: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
To: Oliver Neukum <oneukum@suse.com>, ryder.lee@mediatek.com
Cc: jianjun.wang@mediatek.com, lpieralisi@kernel.org, kw@linux.com,
	robh@kernel.org, bhelgaas@google.com, p.zabel@pengutronix.de,
	matthias.bgg@gmail.com, linux-pci@vger.kernel.org,
	linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] PCI: mediatek-gen3: Stop acquiring spinlocks in {suspend,resume}_noirq
Date: Thu, 8 Jun 2023 12:17:34 +0200	[thread overview]
Message-ID: <6d31e143-1b8f-21ba-8ffe-cee9cac324ea@collabora.com> (raw)
In-Reply-To: <479acc85-2349-d0ac-851c-f57b1bf6aa9e@suse.com>

Il 08/05/23 09:44, Oliver Neukum ha scritto:
> On 04.05.23 13:35, AngeloGioacchino Del Regno wrote:
> 
> Hi,
> 
> looking at your patch I am afraid there is an issue.
> 
>> In mtk_pcie_suspend_noirq() and mtk_pcie_resume_noirq() we are,
>> respectively, disabling and enabling generation of interrupts and
>> then saving and restoring the enabled interrupts register: since
>> we're using noirq PM callbacks, that can be safely done without
>> holding any spin lock.
> 
> Why? You can still race with another CPU in task context.
> That is if you say that you do not need locking to touch
> PCIE_INT_ENABLE_REG that is fine, but then why do you remove
> it from one place only?
> It is also touched in mtk_pcie_probe() at a minimum.
> 
> 
>> That was noticed because of, and solves, the following issue:
>>
>> <4>[   74.185982] ========================================================
>> <4>[   74.192629] WARNING: possible irq lock inversion dependency detected
>> <4>[   74.199276] 6.3.0-next-20230428+ #51 Tainted: G        W
>> <4>[   74.205664] --------------------------------------------------------
>> <4>[   74.212309] systemd-sleep/809 just changed the state of lock:
>> <4>[   74.218347] ffff65a5c34c65a0 (&pcie->irq_lock){+...}-{2:2}, at: 
>> mtk_pcie_resume+0x50/0xa8
>> <4>[   74.226870] but this lock was taken by another, HARDIRQ-safe lock in the past:
>> <4>[   74.234389]  (&irq_desc_lock_class){-.-.}-{2:2}
>> <4>[   74.234409]
>> <4>[   74.234409]
>> <4>[   74.234409] and interrupts could create inverse lock ordering between them.
>> <4>[   74.234409]
>> <4>[   74.251704]
>> <4>[   74.251704] other info that might help us debug this:
>> <4>[   74.258785]  Possible interrupt unsafe locking scenario:
>> <4>[   74.258785]
>> <4>[   74.266126]        CPU0                    CPU1
>> <4>[   74.270942]        ----                    ----
>> <4>[   74.275758]   lock(&pcie->irq_lock);
> 
> Lock A
> 
>> <4>[   74.279627]                                local_irq_disable();
> 
> strictly speaking irrelevant
> 
>> <4>[   74.285836]                                lock(&irq_desc_lock_class);
> 
> lock B
> 
>> <4>[   74.292667]                                lock(&pcie->irq_lock);
> 
> lock A
> 
>> <4>[   74.299061]   <Interrupt>
> 
> You do not need that interrupt.
> 
>> <4>[   74.301960]     lock(&irq_desc_lock_class);
> 
> lock B
> 
>> <4>[   74.306438]
>> <4>[   74.306438]  *** DEADLOCK ***

Sorry for the very late reply. I just noticed this.

I'm unsure, at this point, about how to solve this warning; ideas?

Thanks,
Angelo

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