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Date: Wed, 02 Jul 2014 13:40:02 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] ide: fix double free
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To: ChenLiang <chenliang88@huawei.com>
Cc: "kwolf@redhat.com" <kwolf@redhat.com>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, "Huangweidong (C)" <weidong.huang@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "stefanha@redhat.com" <stefanha@redhat.com>

Il 02/07/2014 13:33, ChenLiang ha scritto:
> On 2014/7/2 18:16, Paolo Bonzini wrote:
>
>> Il 02/07/2014 11:46, Gonglei (Arei) ha scritto:
>>> Hi, Paolo. We have tested your above patch, and it works well for us.
>>
>> I'm still not sure where the fix is.  I jotted the patch quickly, but I'd rather understand it better before submitting it.  Here is it again:
>>
>> --- a/dma-helpers.c
>> +++ b/dma-helpers.c
>> @@ -181,15 +181,15 @@ static void dma_aio_cancel(BlockDriverAIOCB *acb)
>>      trace_dma_aio_cancel(dbs);
>>
>> +    dbs->in_cancel = true;
>>      if (dbs->acb) {
>>          BlockDriverAIOCB *acb = dbs->acb;
>>          dbs->acb = NULL;
>> -        dbs->in_cancel = true;
>>          bdrv_aio_cancel(acb);
>> -        dbs->in_cancel = false;
>>      }
>>      dbs->common.cb = NULL;
>>      dma_complete(dbs, 0);
>> +    qemu_aio_release(dbs);
>>  }
>>
>
>
> Hmm, dbs->in_cancel will be true always. Although this will avoid freeing dbs by dma_comlete.
> But it maybe a mistake.

This was on purpose; I'm doing the free myself in dma_aio_cancel, so I 
wanted to avoid the qemu_aio_release from dma_complete.  This was in 
case of a recursive call to dma_complete.  But I don't see how that 
recursive call could happen outside the "if (dbs->acb)"; and inside the 
"if" the protection is there already.

Can you gather the backtraces for _both_ calls to qemu_aio_release, 
rather than just the second?

With what guest are you encountering the bug?

Paolo