[PATCH] block-backend: fix virtio-scsi assertion failure with blk_drain_noref()

[Stefan Hajnoczi <stefanha@redhat.com>]

scsi_device_purge_requests() is called from I/O code by virtio-scsi TMF
emulation code. It must not call Global State APIs like blk_drain()
because that results in an assertion failure.

blk_drain() is a Global State API because it uses bdrv_unref(). Actually
ref/unref is unnecessary in device emulation code because the drive=
qdev property holds the reference.

Introduce blk_drain_noref(), a variant of blk_drain() that is
IO_OR_GS_CODE() and doesn't take a reference to the BlockBackend.

This fixes the following virtio-scsi IOThread assertion failure when
hot-plugging scsi-hd devices:

  qemu-kvm: ../block/block-backend.c:1780: void blk_drain(BlockBackend *): Assertion `qemu_in_main_thread()' failed.

  #0  0x00007f4b6a0a154c __pthread_kill_implementation (libc.so.6>
  #1  0x00007f4b6a054d46 raise (libc.so.6 + 0x54d46)
  #2  0x00007f4b6a0287f3 abort (libc.so.6 + 0x287f3)
  #3  0x00007f4b6a02871b __assert_fail_base.cold (libc.so.6 + 0x2>
  #4  0x00007f4b6a04dce6 __assert_fail (libc.so.6 + 0x4dce6)
  #5  0x000055c811253ac0 blk_drain (qemu-kvm + 0x808ac0)
  #6  0x000055c810ecb951 scsi_device_purge_requests (qemu-kvm + 0>
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^
  #7  0x000055c810ed6ab8 scsi_disk_reset (qemu-kvm + 0x48bab8)
  #8  0x000055c8111dc908 resettable_phase_hold (qemu-kvm + 0x7919>
  #9  0x000055c8111db7f0 device_cold_reset (qemu-kvm + 0x7907f0)
  #10 0x000055c8110bb1a6 virtio_scsi_handle_ctrl (qemu-kvm + 0x67>
  #11 0x000055c8110d458f virtio_queue_host_notifier_read (qemu-kv>
  #12 0x000055c811404a73 aio_dispatch_handler (qemu-kvm + 0x9b9a7>
  #13 0x000055c811405743 aio_poll (qemu-kvm + 0x9ba743)
  #14 0x000055c81121d532 iothread_run (qemu-kvm + 0x7d2532)
  #15 0x000055c81140926a qemu_thread_start (qemu-kvm + 0x9be26a)
  #16 0x00007f4b6a09f802 start_thread (libc.so.6 + 0x9f802)
  #17 0x00007f4b6a03f450 __clone3 (libc.so.6 + 0x3f450)

Reported-by: Qing Wang <qinwang@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2155748
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/sysemu/block-backend-io.h |  1 +
 block/block-backend.c             | 23 +++++++++++++++++++++++
 hw/scsi/scsi-bus.c                |  2 +-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
index 7ec6d978d4..f73db1f77c 100644
--- a/include/sysemu/block-backend-io.h
+++ b/include/sysemu/block-backend-io.h
@@ -73,6 +73,7 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
 int blk_get_max_iov(BlockBackend *blk);
 int blk_get_max_hw_iov(BlockBackend *blk);
 
+void blk_drain_noref(BlockBackend *blk);
 void blk_io_plug(BlockBackend *blk);
 void blk_io_unplug(BlockBackend *blk);
 AioContext *blk_get_aio_context(BlockBackend *blk);
diff --git a/block/block-backend.c b/block/block-backend.c
index ba7bf1d6bc..20914bae7b 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1815,6 +1815,29 @@ void blk_drain(BlockBackend *blk)
     }
 }
 
+/*
+ * Same as blk_drain() but the caller must hold a reference to blk. May be
+ * called from Global State or I/O API code. Device emulation can call this
+ * because the qdev drive= property holds the reference.
+ */
+void blk_drain_noref(BlockBackend *blk)
+{
+    BlockDriverState *bs = blk_bs(blk);
+    IO_OR_GS_CODE();
+
+    if (bs) {
+        bdrv_drained_begin(bs);
+    }
+
+    /* We may have -ENOMEDIUM completions in flight */
+    AIO_WAIT_WHILE(blk_get_aio_context(blk),
+                   qatomic_mb_read(&blk->in_flight) > 0);
+
+    if (bs) {
+        bdrv_drained_end(bs);
+    }
+}
+
 void blk_drain_all(void)
 {
     BlockBackend *blk = NULL;
diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index ceceafb2cd..110fd85106 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -1663,7 +1663,7 @@ void scsi_device_purge_requests(SCSIDevice *sdev, SCSISense sense)
         req = QTAILQ_FIRST(&sdev->requests);
         scsi_req_cancel_async(req, NULL);
     }
-    blk_drain(sdev->conf.blk);
+    blk_drain_noref(sdev->conf.blk);
     aio_context_release(blk_get_aio_context(sdev->conf.blk));
     scsi_device_set_ua(sdev, sense);
 }
-- 
2.39.0