Xen patch

- Fix Xen backend block detach via xenstore.
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Merge remote-tracking branch 'remotes/aperard/tags/pull-xen-20210323' into staging

Xen patch

- Fix Xen backend block detach via xenstore.

# gpg: Signature made Tue 23 Mar 2021 11:53:08 GMT
# gpg:                using RSA key F80C006308E22CFD8A92E7980CF5572FD7FB55AF
# gpg: Good signature from "Anthony PERARD <anthony.perard@gmail.com>" [marginal]
# gpg:                 aka "Anthony PERARD <anthony.perard@citrix.com>" [marginal]
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# Primary key fingerprint: 5379 2F71 024C 600F 778A  7161 D8D5 7199 DF83 42C8
#      Subkey fingerprint: F80C 0063 08E2 2CFD 8A92  E798 0CF5 572F D7FB 55AF

* remotes/aperard/tags/pull-xen-20210323:
  xen-block: Fix removal of backend instance via xenstore

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/block/xen-block.c

@@ -972,6 +972,15 @@ static void xen_block_device_destroy(XenBackendInstance *backend,
 
     object_unparent(OBJECT(xendev));
 
+    /*
+     * Drain all pending RCU callbacks as object_unparent() frees `xendev'
+     * in a RCU callback.
+     * And due to the property "drive" still existing in `xendev', we
+     * can't destroy the XenBlockDrive associated with `xendev' with
+     * xen_block_drive_destroy() below.
+     */
+    drain_call_rcu();
+
     if (iothread) {
         xen_block_iothread_destroy(iothread, errp);
         if (*errp) {