linux-kernel - [PATCH] dmaengine: mediatek: Fix deadlock caused by synchronize

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Date:   Sun,  6 Aug 2023 11:25:11 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     sean.wang@...iatek.com
Cc:     vkoul@...nel.org, matthias.bgg@...il.com,
        angelogioacchino.delregno@...labora.com, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH] dmaengine: mediatek: Fix deadlock caused by synchronize_irq()

The synchronize_irq(c->irq) will not return until the IRQ handler
mtk_uart_apdma_irq_handler() is completed. If the synchronize_irq()
holds a spin_lock and waits the IRQ handler to complete, but the
IRQ handler also needs the same spin_lock. The deadlock will happen.
The process is shown below:

          cpu0                        cpu1
mtk_uart_apdma_device_pause() | mtk_uart_apdma_irq_handler()
  spin_lock_irqsave()         |
                              |   spin_lock_irqsave()
  //hold the lock to wait     |
  synchronize_irq()           |

This patch reorders the synchronize_irq(c->irq) outside the spin_lock
in order to mitigate the bug.

Fixes: 9135408c3ace ("dmaengine: mediatek: Add MediaTek UART APDMA support")
Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
 drivers/dma/mediatek/mtk-uart-apdma.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/dma/mediatek/mtk-uart-apdma.c b/drivers/dma/mediatek/mtk-uart-apdma.c
index a1517ef1f4a..0acf6a92a4a 100644
--- a/drivers/dma/mediatek/mtk-uart-apdma.c
+++ b/drivers/dma/mediatek/mtk-uart-apdma.c
@@ -451,9 +451,8 @@ static int mtk_uart_apdma_device_pause(struct dma_chan *chan)
 	mtk_uart_apdma_write(c, VFF_EN, VFF_EN_CLR_B);
 	mtk_uart_apdma_write(c, VFF_INT_EN, VFF_INT_EN_CLR_B);
 
-	synchronize_irq(c->irq);
-
 	spin_unlock_irqrestore(&c->vc.lock, flags);
+	synchronize_irq(c->irq);
 
 	return 0;
 }
-- 
2.17.1