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From c800bff089dec124e622397583abfc28308d1c42 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Thu, 12 Jul 2018 16:06:19 +0200
Subject: [PATCH 215/268] block: fix QEMU crash with scsi-hd and drive_del

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20180712160619.30712-2-kwolf@redhat.com>
Patchwork-id: 81334
O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 1/1] block: fix QEMU crash with scsi-hd and drive_del
Bugzilla: 1599515
RH-Acked-by: Fam Zheng <famz@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

From: Greg Kurz <groug@kaod.org>

Removing a drive with drive_del while it is being used to run an I/O
intensive workload can cause QEMU to crash.

An AIO flush can yield at some point:

blk_aio_flush_entry()
 blk_co_flush(blk)
  bdrv_co_flush(blk->root->bs)
   ...
    qemu_coroutine_yield()

and let the HMP command to run, free blk->root and give control
back to the AIO flush:

    hmp_drive_del()
     blk_remove_bs()
      bdrv_root_unref_child(blk->root)
       child_bs = blk->root->bs
       bdrv_detach_child(blk->root)
        bdrv_replace_child(blk->root, NULL)
         blk->root->bs = NULL
        g_free(blk->root) <============== blk->root becomes stale
       bdrv_unref(child_bs)
        bdrv_delete(child_bs)
         bdrv_close()
          bdrv_drained_begin()
           bdrv_do_drained_begin()
            bdrv_drain_recurse()
             aio_poll()
              ...
              qemu_coroutine_switch()

and the AIO flush completion ends up dereferencing blk->root:

  blk_aio_complete()
   scsi_aio_complete()
    blk_get_aio_context(blk)
     bs = blk_bs(blk)
 ie, bs = blk->root ? blk->root->bs : NULL
            ^^^^^
            stale

The problem is that we should avoid making block driver graph
changes while we have in-flight requests. Let's drain all I/O
for this BB before calling bdrv_root_unref_child().

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit f45280cbf66d8e58224f6a253d0ae2aa72cc6280)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 block/block-backend.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index 5562ec4..f34e4c3 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -768,6 +768,11 @@ void blk_remove_bs(BlockBackend *blk)
 
     blk_update_root_state(blk);
 
+    /* bdrv_root_unref_child() will cause blk->root to become stale and may
+     * switch to a completion coroutine later on. Let's drain all I/O here
+     * to avoid that and a potential QEMU crash.
+     */
+    blk_drain(blk);
     bdrv_root_unref_child(blk->root);
     blk->root = NULL;
 }
-- 
1.8.3.1