[Qemu-devel] [PATCH] block: Fix bdrv_drain in coroutine

[Fam Zheng <famz@redhat.com>]

Using the nested aio_poll() in coroutine is a bad idea. This patch
replaces the aio_poll loop in bdrv_drain with a BH, if called in
coroutine.

For example, the bdrv_drain() in mirror.c can hang when a guest issued
request is pending on it in qemu_co_mutex_lock().

Mirror coroutine in this case has just finished a request, and the block
job is about to complete. It calls bdrv_drain() which waits for the
other coroutine to complete. The other coroutine is a scsi-disk request.
The deadlock happens when the latter is in turn pending on the former to
yield/terminate, qemu_co_mutex_lock(). The state flow is as below
(assuming a qcow2 image):

  mirror coroutine               scsi-disk coroutine
  -------------------------------------------------------------
  do last write

    qcow2:qemu_co_mutex_lock()
    ...
                                 scsi disk read

                                   tracked request begin

                                   qcow2:qemu_co_mutex_lock.enter

    qcow2:qemu_co_mutex_unlock()

  bdrv_drain
    while (has tracked request)
      aio_poll()

In the scsi-disk coroutine, the qemu_co_mutex_lock() will never return
because the mirror coroutine is blocked in the aio_poll(blocking=true).

Reported-by: Laurent Vivier <lvivier@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 block/io.c | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/block/io.c b/block/io.c
index c4869b9..d0a4551 100644
--- a/block/io.c
+++ b/block/io.c
@@ -253,6 +253,42 @@ static void bdrv_drain_recurse(BlockDriverState *bs)
     }
 }
 
+typedef struct {
+    Coroutine *co;
+    BlockDriverState *bs;
+    bool done;
+} BdrvCoDrainData;
+
+static void bdrv_co_drain_bh_cb(void *opaque)
+{
+    BdrvCoDrainData *data = opaque;
+    Coroutine *co = data->co;
+
+    bdrv_drain(data->bs);
+    data->done = true;
+    qemu_coroutine_enter(co, NULL);
+}
+
+static void coroutine_fn bdrv_co_drain(BlockDriverState *bs)
+{
+    QEMUBH *bh;
+    BdrvCoDrainData data;
+
+    assert(qemu_in_coroutine());
+    data = (BdrvCoDrainData) {
+        .co = qemu_coroutine_self(),
+        .bs = bs,
+        .done = false,
+    };
+    bh = aio_bh_new(bdrv_get_aio_context(bs), bdrv_co_drain_bh_cb, &data),
+    qemu_bh_schedule(bh);
+
+    do {
+        qemu_coroutine_yield();
+    } while (!data.done);
+    qemu_bh_delete(bh);
+}
+
 /*
  * Wait for pending requests to complete on a single BlockDriverState subtree,
  * and suspend block driver's internal I/O until next request arrives.
@@ -269,6 +305,10 @@ void bdrv_drain(BlockDriverState *bs)
     bool busy = true;
 
     bdrv_drain_recurse(bs);
+    if (qemu_in_coroutine()) {
+        bdrv_co_drain(bs);
+        return;
+    }
     while (busy) {
         /* Keep iterating */
          bdrv_flush_io_queue(bs);
-- 
2.7.4