Re: [RESEND PATCH] hw/dma: fix crash caused by race condition

[David Hildenbrand <david@redhat.com>]

On 27.04.22 22:51, Tong Zhang wrote:
> assert(dbs->acb) is meant to check the return value of io_func per
> documented in commit 6bee44ea34 ("dma: the passed io_func does not
> return NULL"). However, there is a chance that after calling
> aio_context_release(dbs->ctx); the dma_blk_cb function is called before
> the assertion and dbs->acb is set to NULL again at line 121. Thus when
> we run assert at line 181 it will fail.
> 
>   softmmu/dma-helpers.c:181: dma_blk_cb: Assertion `dbs->acb' failed.
> 
> Reported-by: Francisco Londono <f.londono@samsung.com>
> Signed-off-by: Tong Zhang <t.zhang2@samsung.com>
> ---
>  softmmu/dma-helpers.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
> index 7820fec54c..cb81017928 100644
> --- a/softmmu/dma-helpers.c
> +++ b/softmmu/dma-helpers.c
> @@ -177,8 +177,8 @@ static void dma_blk_cb(void *opaque, int ret)
>      aio_context_acquire(dbs->ctx);
>      dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,
>                              dma_blk_cb, dbs, dbs->io_func_opaque);
> -    aio_context_release(dbs->ctx);
>      assert(dbs->acb);
> +    aio_context_release(dbs->ctx);
>  }
>  
>  static void dma_aio_cancel(BlockAIOCB *acb)

I'm fairly new to that code, but I wonder what prevents dma_blk_cb() to
run after you reshuffled the code?

After all, acquire/release is only around the dbs->io_func() call, so I
don't immediately see how it interacts with re-entrance?

-- 
Thanks,

David / dhildenb