[Qemu-devel] [PULL 1/3] dma-helpers: ensure AIO callback is invoked after cancellation

[John Snow <jsnow@redhat.com>]

From: Paolo Bonzini <pbonzini@redhat.com>

dma_aio_cancel unschedules the BH if there is one, which corresponds
to the reschedule_dma case of dma_blk_cb.  This can stall the DMA
permanently, because dma_complete will never get invoked and therefore
nobody will ever invoke the original AIO callback in dbs->common.cb.

Fix this by invoking the callback (which is ensured to happen after
a bdrv_aio_cancel_async, or done manually in the dbs->bh case), and
add assertions to check that the DMA state machine is indeed waiting
for dma_complete or reschedule_dma, but never both.

Reported-by: John Snow <jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20190729213416.1972-1-pbonzini@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
---
 dma-helpers.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/dma-helpers.c b/dma-helpers.c
index 2d7e02d35e5..d3871dc61ea 100644
--- a/dma-helpers.c
+++ b/dma-helpers.c
@@ -90,6 +90,7 @@ static void reschedule_dma(void *opaque)
 {
     DMAAIOCB *dbs = (DMAAIOCB *)opaque;
 
+    assert(!dbs->acb && dbs->bh);
     qemu_bh_delete(dbs->bh);
     dbs->bh = NULL;
     dma_blk_cb(dbs, 0);
@@ -111,15 +112,12 @@ static void dma_complete(DMAAIOCB *dbs, int ret)
 {
     trace_dma_complete(dbs, ret, dbs->common.cb);
 
+    assert(!dbs->acb && !dbs->bh);
     dma_blk_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;
-    }
     qemu_aio_unref(dbs);
 }
 
@@ -179,14 +177,21 @@ static void dma_aio_cancel(BlockAIOCB *acb)
 
     trace_dma_aio_cancel(dbs);
 
+    assert(!(dbs->acb && dbs->bh));
     if (dbs->acb) {
+        /* This will invoke dma_blk_cb.  */
         blk_aio_cancel_async(dbs->acb);
+        return;
     }
+
     if (dbs->bh) {
         cpu_unregister_map_client(dbs->bh);
         qemu_bh_delete(dbs->bh);
         dbs->bh = NULL;
     }
+    if (dbs->common.cb) {
+        dbs->common.cb(dbs->common.opaque, -ECANCELED);
+    }
 }
 
 static AioContext *dma_get_aio_context(BlockAIOCB *acb)
-- 
2.21.0