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Date: Mon, 31 Oct 2011 17:00:44 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] dma: Avoid reentrancy in DMA transfer
	handlers
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

Am 31.10.2011 16:34, schrieb Paolo Bonzini:
> On 10/31/2011 03:46 PM, Kevin Wolf wrote:
>>>  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.
>>
>> I was about to agree, but in fact adding a rearm = 1; line leads to
>> crashes, whereas in the version I posted it just works. So it looks like
>> something is wrong with doing it, even though it seemed to make perfect
>> sense at the first sight.
> 
> But what will restart the DMA at the end of the synchronous I/O, then?

bdrv_read/write are called inside fdctrl_read_data(), so the outer
DMA_run() already has rearm = 1.

I think the more interesting question is why rescheduling can break
anything. Where would we schedule the BH additionally when it isn't
already scheduled anyway?

Kevin