Re: [PATCH] dmaengine: shdma: fix a race condition in __ld_cleanup()

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Shimoda-san,

On Thursday 09 October 2014 11:09:00 Yoshihiro Shimoda wrote:
> This patch fixes a race condition about a list of shdma-base driver.
> If we don't apply this patch, a dma slave driver (especially a usb
> peripheral driver) may not be able to start the transfer.
> 
> If a dma slave driver has a callback, __ld_cleanup() will call
> the callback before this driver removes the list. After the callback,
> since the return value of __ld_cleanup() is not zero,
> shdma_chan_ld_cleanup() calls __ld_cleanup() again. And, __ld_clean()
> will removes the list.
> 
> At this time, if a dma slave driver calls dmaengine_submit() before
> this driver removes the list, this driver will set schan->pm_state
> to SHDMA_PM_PENDING in shdma_tx_submit(). And then, even if a dma
> slave driver calls dma_async_issue_pending(), this driver don't
> start the transfer because the schan->pm_state is SHDMA_PM_PENDING
> in shdma_issue_pending().
> 
> So, this patch adds a new condition in __ld_clean() to check if the
> schan->pm_state is SHDMA_PM_PENDING or not.
> 
> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>

This looks good to me, but given the complexity of the shdma-base code it's 
very difficult to make sure there will be no undesired side effect. I don't 
have any issue with the patch, but I can't ack it either.

> ---
>  This patch is based on slave-dma.git / next branch.
>  (commit id : ade4a2a8e6e40fe18be1f83e5ea2e915692a0028)
> 
>  drivers/dma/sh/shdma-base.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/dma/sh/shdma-base.c b/drivers/dma/sh/shdma-base.c
> index 42d4974..2fd2f0ea 100644
> --- a/drivers/dma/sh/shdma-base.c
> +++ b/drivers/dma/sh/shdma-base.c
> @@ -391,6 +391,8 @@ static dma_async_tx_callback __ld_cleanup(struct
> shdma_chan *schan, bool all) dev_dbg(schan->dev, "Bring down channel %d\n",
> schan->id);
>  				pm_runtime_put(schan->dev);
>  				schan->pm_state = SHDMA_PM_ESTABLISHED;
> +			} else if (schan->pm_state = SHDMA_PM_PENDING) {
> +				shdma_chan_xfer_ld_queue(schan);
>  			}
>  		}
>  	}

-- 
Regards,

Laurent Pinchart