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@zju.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

On Sun, 06 Aug 2023 11:25:11 +0800, Duoming Zhou wrote:
> 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()           |
> 
> [...]

Applied, thanks!

[1/1] dmaengine: mediatek: Fix deadlock caused by synchronize_irq()
      commit: 01f1ae2733e2bb4de92fefcea5fda847d92aede1

Best regards,
-- 
~Vinod

On 8/6/23 06:25, Duoming Zhou wrote:
> 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@zju.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;
>   }


Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>