Date:   Tue, 14 Nov 2017 16:47:07 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     <vinod.koul@...el.com>, Russell King <linux@...linux.org.uk>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <lars@...afoo.de>
Subject: Re: [PATCH 00/10] dmaengine: fix race with vchan_complete

Hi,

Russell, I have missed you from the To: list of this series by mistake,
I'm sorry about that.

- Péter

On 2017-11-14 16:32, Peter Ujfalusi wrote:
> Hi,
> 
> With the introduction of .device_synchronize callback it was thought that the
> race caused crash observed in vchan_complete is fixed, but unfortunately it can
> still happen.
> 
> The observed scenario (really hard to reproduce) is:
> Cyclic mode
> - DMA period interrupt
>  - call to vchan_cyclic_callback() which sets vc->cyclic to vd and schedules the
>    vchan_complete tasklet
> - .terminate_all is called
>  - we make sure that no further DMA irqs are going to be handled, but the
>    tasklet had been already scheduled
>  - we free up the descriptor to avoid leaking memory
> - the vchan_complete tasklet starts to execute and it checks vc->cyclic, which
>   is not NULL, saves the vd pointer (which points to an already freed up memory)
>   and try to access it later to call the callback
> - the tasklet_kill() in .device_synchronize will make sure that the tasklet is
>   going to finish it's execution if it is already scheduled, it can only help if
>   the tasklet is yet to be executed.
> 
> At this point it is just matter of luck if the vc->cyclic is still pointing to
> an unchanged memory location or it is taken into use and thus it is corrupted.
> 
> My first approach was to just set vc->cyclic to NULL in the .terminate_all
> callback, but that still have theoritical race: if the vchan_complete is
> executing and it saves the vd from vc->cyclic (protected by the vc->lock). If at
> that point the .terminate_all is called it will wait for the lock and start
> executing. _if_ the .terminate_all free up the vd before the vchan_complete
> reaches the point when it is going to call the callback, then we have the race.
> 
> The series will do this:
> - the drivers should call vchan_terminate_vdesc() instead of directly freeing up
>   the descriptor. vchan_terminate_vdesc() will save the vd as vc->vd_terminated
>   and will set the vc->cyclic to NULL, all while holding the lock.
> - the drivers must implement the .device_synchronize callback and within the
>   vchan_synchronize() we free up the vc->vd_terminated after we killed the
>   tasklet.
> 
> I have tested this on platforms using TI's eDMA and sDMA and have not seen any
> side effect so far and a client tested similar set on a setup where it was easy
> to reproduce the race.
> 
> By looking for similar patterns in other drivers I have implemented the fix for
> the ones where it looked straight forward.
> 
> Regards,
> Peter
> ---
> Peter Ujfalusi (10):
>   dmaengine: virt-dma: Add helper to free/reuse a descriptor
>   dmaengine: virt-dma: Support for race free transfer termination
>   dmaengine: omap-dma: Use vchan_terminate_vdesc() instead of desc_free
>   dmaengine: edma: Use vchan_terminate_vdesc() instead of desc_free
>   dmaengine: bcm2835-dma: Use vchan_terminate_vdesc() instead of
>     desc_free
>   dmaengine: dma-jz4780: Use vchan_terminate_vdesc() instead of
>     desc_free
>   dmaengine: amba-pl08x: Use vchan_terminate_vdesc() instead of
>     desc_free
>   dmaengine: img-mdc-dma: Use vchan_terminate_vdesc() instead of
>     desc_free
>   dmaengine: k3dma: Use vchan_terminate_vdesc() instead of desc_free
>   dmaengine: s3c24xx-dma: Use vchan_terminate_vdesc() instead of
>     desc_free
> 
>  drivers/dma/amba-pl08x.c  | 11 ++++++++++-
>  drivers/dma/bcm2835-dma.c | 10 +++++++++-
>  drivers/dma/dma-jz4780.c  | 10 +++++++++-
>  drivers/dma/edma.c        |  7 ++-----
>  drivers/dma/img-mdc-dma.c | 17 ++++++++++++-----
>  drivers/dma/k3dma.c       | 10 +++++++++-
>  drivers/dma/omap-dma.c    |  2 +-
>  drivers/dma/s3c24xx-dma.c | 11 ++++++++++-
>  drivers/dma/virt-dma.c    |  5 +----
>  drivers/dma/virt-dma.h    | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  10 files changed, 107 insertions(+), 20 deletions(-)
> 

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