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

[Tong Zhang <t.zhang2@samsung.com>]

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)
-- 
2.25.1