Re: [Qemu-devel] [PATCH] dma: Avoid reentrancy in DMA transfer handlers

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/28/2011 04:18 PM, Kevin Wolf wrote:
> With the conversion of the block layer to coroutines, bdrv_read/write
> have changed to run a nested event loop that calls qemu_bh_poll.
> Consequently a scheduled BH can be called while a DMA transfer handler
> runs and this means that DMA_run becomes reentrant.
>
> Devices haven't been designed to cope with that, so instead of running a
> nested transfer handler just wait for the next invocation of the BH from the
> main loop.
>
> This fixes some problems with the floppy device.
>
> Signed-off-by: Kevin Wolf<kwolf@redhat.com>
> ---
>   hw/dma.c |    9 +++++++++
>   1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/hw/dma.c b/hw/dma.c
> index 8a7302a..e8d6341 100644
> --- a/hw/dma.c
> +++ b/hw/dma.c
> @@ -358,6 +358,13 @@ static void DMA_run (void)
>       struct dma_cont *d;
>       int icont, ichan;
>       int rearm = 0;
> +    static int running = 0;
> +
> +    if (running) {
> +        goto out;
> +    } else {
> +        running = 1;
> +    }
>
>       d = dma_controllers;
>
> @@ -374,6 +381,8 @@ static void DMA_run (void)
>           }
>       }
>
> +out:
> +    running = 0;
>       if (rearm)
>           qemu_bh_schedule_idle(dma_bh);
>   }

Hmm, I think you should set rearm = 1 to ensure the BH is run when 
ultimately you leave the sync read.  Sorry for not spotting this before.

Paolo