[PATCH RESEND] serial: imx: Fix potential deadlock on sport->port.lock

[Uwe Kleine-König]

Hello Vinod,

thanks for your quick answer!

On Thu, Sep 28, 2023 at 01:08:15PM +0530, Vinod Koul wrote:
> On 28-09-23, 08:07, Uwe Kleine-König wrote:
> > [Cc += Vinod Koul, dmaengine at vger.kernel.org]
> > 
> > Hello,
> > 
> > On Wed, Sep 27, 2023 at 06:19:39PM +0000, Chengfeng Ye wrote:
> > > As &sport->port.lock is acquired under irq context along the following
> > > call chain from imx_uart_rtsint(), other acquisition of the same lock
> > > inside process context or softirq context should disable irq avoid double
> > > lock.
> > > 
> > > <deadlock #1>
> > > 
> > > imx_uart_dma_rx_callback()
> > > --> spin_lock(&sport->port.lock)
> > > <interrupt>
> > >    --> imx_uart_rtsint()
> > >    --> spin_lock(&sport->port.lock)
> > > 
> > > This flaw was found by an experimental static analysis tool I am
> > > developing for irq-related deadlock.
> > 
> > Ah, I understood before that you really experienced that deadlock (or a
> > lockdep splat). I didn't test anything, but I think the
> > imx_uart_dma_rx_callback() is called indirectly by
> > sdma_update_channel_loop() which is called in irq context. I don't know
> > if this is the case for all dma drivers?!
> > 
> > @Vinod: Maybe you can chime in here: Is a dma callback always called in
> > irq context?
> 
> Not in callback but a tasklet context. The DMA irq handler is supposed
> to use a tasklet for invoking the callback

So drivers/dma/imx-sdma.c is bogous as it calls

	-> sdma_int_handler()
	  -> sdma_update_channel_loop()
	    -> dmaengine_desc_get_callback_invoke()

resulting in imx_uart_dma_rx_callback() (and others) being called in irq
context, right?

In that case:

Acked-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>

(for the imx-UART patch that stops assuming imx_uart_dma_rx_callback()
is called with irqs off).

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |
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