linux-kernel - Re: [PATCH v2] dmaengine: mediatek: Fix a possible deadlock error in mtk_cqdma_tx

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Message-ID: <fede9d2d-6225-4760-8920-c5963665ca1e@collabora.com>
Date: Thu, 8 May 2025 10:06:13 +0200
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Qiu-ji Chen <chenqiuji666@...il.com>, sean.wang@...iatek.com,
 vkoul@...nel.org, matthias.bgg@...il.com
Cc: dmaengine@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
 baijiaju1990@...il.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] dmaengine: mediatek: Fix a possible deadlock error in
 mtk_cqdma_tx_status()

Il 08/05/25 09:36, Qiu-ji Chen ha scritto:
> Fix a potential deadlock bug. Observe that in the mtk-cqdma.c
> file, functions like mtk_cqdma_issue_pending() and
> mtk_cqdma_free_active_desc() properly acquire the pc lock before the vc
> lock when handling pc and vc fields. However, mtk_cqdma_tx_status()
> violates this order by first acquiring the vc lock before invoking
> mtk_cqdma_find_active_desc(), which subsequently takes the pc lock. This
> reversed locking sequence (vc → pc) contradicts the established
> pc → vc order and creates deadlock risks.
> 
> Fix the issue by moving the vc lock acquisition code from
> mtk_cqdma_find_active_desc() to mtk_cqdma_tx_status(). Ensure the pc lock
> is acquired before the vc lock in the calling function to maintain correct
> locking hierarchy. Note that since mtk_cqdma_find_active_desc() is a
> static function with only one caller (mtk_cqdma_tx_status()), this
> modification safely eliminates the deadlock possibility without affecting
> other components.
> 
> This possible bug is found by an experimental static analysis tool
> developed by our team. This tool analyzes the locking APIs to extract
> function pairs that can be concurrently executed, and then analyzes the
> instructions in the paired functions to identify possible concurrency bugs
> including deadlocks, data races and atomicity violations.
> 
> Fixes: b1f01e48df5a ("dmaengine: mediatek: Add MediaTek Command-Queue DMA controller for MT6765 SoC")
> Cc: stable@...r.kernel.org
> Signed-off-by: Qiu-ji Chen <chenqiuji666@...il.com>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>