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Date: Wed, 02 Jul 2014 11:04:04 +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: arei.gonglei@huawei.com, qemu-devel@nongnu.org
Cc: kwolf@redhat.com, Chenliang <chenliang88@huawei.com>, weidong.huang@huawei.com, stefanha@redhat.com

Il 02/07/2014 10:50, arei.gonglei@huawei.com ha scritto:
>      if (bus->dma->aiocb) {
> -#ifdef DEBUG_AIO
> -        printf("aio_cancel\n");
> -#endif
> -        bdrv_aio_cancel(bus->dma->aiocb);
> -        bus->dma->aiocb = NULL;
> +        bdrv_drain_all();
> +        bdrv_flush_all();
> +        assert(bus->dma->aiocb == NULL);
>      }

This is definitely a heavyweight solution, and in fact the bug should 
not be there in the first place.  See dma_complete:

static void dma_complete(DMAAIOCB *dbs, int ret)
{
    trace_dma_complete(dbs, ret, dbs->common.cb);

    dma_bdrv_unmap(dbs);
    if (dbs->common.cb) {
        dbs->common.cb(dbs->common.opaque, ret);
    }
    qemu_iovec_destroy(&dbs->iov);
    if (dbs->bh) {
        qemu_bh_delete(dbs->bh);
        dbs->bh = NULL;
    }
    if (!dbs->in_cancel) {
        /* Requests may complete while dma_aio_cancel is in progress.  In
         * this case, the AIOCB should not be released because it is still
         * referenced by dma_aio_cancel.  */
        qemu_aio_release(dbs);
    }
}

Perhaps something like this?

diff --git a/dma-helpers.c b/dma-helpers.c
index 53cbe92..21b70d12 100644
--- 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);
 }
 
 static const AIOCBInfo dma_aiocb_info = {