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* [Qemu-devel] [PATCH v4] block: fix QEMU crash with scsi-hd and drive_del
@ 2018-05-28 12:03 Greg Kurz
  2018-05-29 18:15 ` Kevin Wolf
                   ` (2 more replies)
  0 siblings, 3 replies; 8+ messages in thread
From: Greg Kurz @ 2018-05-28 12:03 UTC (permalink / raw)
  To: qemu-devel
  Cc: Kevin Wolf, Max Reitz, Stefan Hajnoczi, Paolo Bonzini,
	qemu-stable

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>
---
v4: - call blk_drain() in blk_remove_bs() (Kevin)

v3: - start drained section before modifying the graph (Stefan)

v2: - drain I/O requests when detaching the BDS (Stefan, Paolo)
---
 block/block-backend.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index 89f47b00ea24..bee1f0e41461 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;
 }

^ permalink raw reply related	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2018-07-23 13:48 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2018-05-28 12:03 [Qemu-devel] [PATCH v4] block: fix QEMU crash with scsi-hd and drive_del Greg Kurz
2018-05-29 18:15 ` Kevin Wolf
2018-05-29 20:19 ` Kevin Wolf
2018-05-29 21:41   ` Greg Kurz
2018-07-18 21:07   ` [Qemu-devel] [Qemu-stable] " Michael Roth
2018-07-23 13:48     ` Kevin Wolf
2018-06-26 19:30 ` [Qemu-devel] " Max Reitz
2018-06-26 19:33   ` Max Reitz

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