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[Bug 1805256] Autopkgtest regression report (qemu/1:2.11+dfsg-1ubuntu7.3

From: Ubuntu SRU Bot
Subject: [Bug 1805256] Autopkgtest regression report (qemu/1:2.11+dfsg-1ubuntu7.30)
Date: Fri, 07 Aug 2020 14:41:56 -0000

All autopkgtests for the newly accepted qemu (1:2.11+dfsg-1ubuntu7.30) for 
bionic have finished running.
The following regressions have been reported in tests triggered by the package:

systemd/237-3ubuntu10.42 (i386)
ubuntu-image/1.9+18.04ubuntu1 (arm64, armhf, i386, s390x, ppc64el, amd64)
cinder/2:12.0.9-0ubuntu1.2 (s390x)

Please visit the excuses page listed below and investigate the failures, 
proceeding afterwards as per the StableReleaseUpdates policy regarding 
autopkgtest regressions [1].


[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

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  qemu-img hangs on rcu_call_ready_event logic in Aarch64 when
  converting images

Status in kunpeng920:
Status in kunpeng920 ubuntu-18.04 series:
Status in kunpeng920 ubuntu-18.04-hwe series:
Status in kunpeng920 ubuntu-19.10 series:
  Fix Released
Status in kunpeng920 ubuntu-20.04 series:
  Fix Released
Status in kunpeng920 upstream-kernel series:
Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Bionic:
  Fix Committed
Status in qemu source package in Eoan:
  Fix Released
Status in qemu source package in Focal:
  Fix Released

Bug description:
  SRU TEAM REVIEWER: This has already been SRUed for Focal, Eoan and Bionic. 
Unfortunately the Bionic SRU did not work and we had to reverse the change. 
Since then we had another update and now I'm retrying the SRU.

  After discussing with @paelzer (and @dannf as a reviewer) extensively,
  Christian and I agreed that we should scope this SRU as Aarch64 only
  AND I was much, much more conservative in question of what is being
  changed in the AIO qemu code.

  New code has been tested against the initial Test Case and the new
  one, regressed for Bionic. More information (about tests and
  discussion) can be found in the MR at




  * QEMU locking primitives might face a race condition in QEMU Async
  I/O bottom halves scheduling. This leads to a dead lock making either
  QEMU or one of its tools to hang indefinitely.

  [Test Case]


  * qemu-img convert -f qcow2 -O qcow2 ./disk01.qcow2 ./output.qcow2

  Hangs indefinitely approximately 30% of the runs in Aarch64.

  [Regression Potential]

  * This is a change to a core part of QEMU: The AIO scheduling. It
  works like a "kernel" scheduler, whereas kernel schedules OS tasks,
  the QEMU AIO code is responsible to schedule QEMU coroutines or event
  listeners callbacks.

  * There was a long discussion upstream about primitives and Aarch64.
  After quite sometime Paolo released this patch and it solves the
  issue. Tested platforms were: amd64 and aarch64 based on his commit

  * Christian suggests that this fix stay little longer in -proposed to
  make sure it won't cause any regressions.

  * dannf suggests we also check for performance regressions; e.g. how
  long it takes to convert a cloud image on high-core systems.



  [Other Info]

   * Original Description bellow:


  qemu-img convert -f qcow2 -O qcow2 ./disk01.qcow2 ./output.qcow2

  Hangs indefinitely approximately 30% of the runs.



  qemu-img convert -m 1 -f qcow2 -O qcow2 ./disk01.qcow2 ./output.qcow2

  Run "qemu-img convert" with "a single coroutine" to avoid this issue.


  (gdb) thread 1
  (gdb) bt
  #0 0x0000ffffbf1ad81c in __GI_ppoll
  #1 0x0000aaaaaabcf73c in ppoll
  #2 qemu_poll_ns
  #3 0x0000aaaaaabd0764 in os_host_main_loop_wait
  #4 main_loop_wait

  (gdb) thread 2
  (gdb) bt
  #0 syscall ()
  #1 0x0000aaaaaabd41cc in qemu_futex_wait
  #2 qemu_event_wait (ev=ev@entry=0xaaaaaac86ce8 <rcu_call_ready_event>)
  #3 0x0000aaaaaabed05c in call_rcu_thread
  #4 0x0000aaaaaabd34c8 in qemu_thread_start
  #5 0x0000ffffbf25c880 in start_thread
  #6 0x0000ffffbf1b6b9c in thread_start ()

  (gdb) thread 3
  (gdb) bt
  #0 0x0000ffffbf11aa20 in __GI___sigtimedwait
  #1 0x0000ffffbf2671b4 in __sigwait
  #2 0x0000aaaaaabd1ddc in sigwait_compat
  #3 0x0000aaaaaabd34c8 in qemu_thread_start
  #4 0x0000ffffbf25c880 in start_thread
  #5 0x0000ffffbf1b6b9c in thread_start


  (gdb) run
  Starting program: /usr/bin/qemu-img convert -f qcow2 -O qcow2
  ./disk01.ext4.qcow2 ./output.qcow2

  [New Thread 0xffffbec5ad90 (LWP 72839)]
  [New Thread 0xffffbe459d90 (LWP 72840)]
  [New Thread 0xffffbdb57d90 (LWP 72841)]
  [New Thread 0xffffacac9d90 (LWP 72859)]
  [New Thread 0xffffa7ffed90 (LWP 72860)]
  [New Thread 0xffffa77fdd90 (LWP 72861)]
  [New Thread 0xffffa6ffcd90 (LWP 72862)]
  [New Thread 0xffffa67fbd90 (LWP 72863)]
  [New Thread 0xffffa5ffad90 (LWP 72864)]

  [Thread 0xffffa5ffad90 (LWP 72864) exited]
  [Thread 0xffffa6ffcd90 (LWP 72862) exited]
  [Thread 0xffffa77fdd90 (LWP 72861) exited]
  [Thread 0xffffbdb57d90 (LWP 72841) exited]
  [Thread 0xffffa67fbd90 (LWP 72863) exited]
  [Thread 0xffffacac9d90 (LWP 72859) exited]
  [Thread 0xffffa7ffed90 (LWP 72860) exited]

  <HUNG w/ 3 threads in the stack trace showed before>

  All the tasks left are blocked in a system call, so no task left to call
  qemu_futex_wake() to unblock thread #2 (in futex()), which would unblock
  thread #1 (doing poll() in a pipe with thread #2).

  Those 7 threads exit before disk conversion is complete (sometimes in
  the beginning, sometimes at the end).


  On the HiSilicon D06 system - a 96 core NUMA arm64 box - qemu-img
  frequently hangs (~50% of the time) with this command:

  qemu-img convert -f qcow2 -O qcow2 /tmp/cloudimg /tmp/cloudimg2

  Where "cloudimg" is a standard qcow2 Ubuntu cloud image. This
  qcow2->qcow2 conversion happens to be something uvtool does every time
  it fetches images.

  Once hung, attaching gdb gives the following backtrace:

  (gdb) bt
  #0  0x0000ffffae4f8154 in __GI_ppoll (fds=0xaaaae8a67dc0, 
      timeout=<optimized out>, timeout@entry=0x0, sigmask=0xffffc123b950)
      at ../sysdeps/unix/sysv/linux/ppoll.c:39
  #1  0x0000aaaabbefaf00 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized 
      __fds=<optimized out>) at /usr/include/aarch64-linux-gnu/bits/poll2.h:77
  #2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>,
      timeout=timeout@entry=-1) at util/qemu-timer.c:322
  #3  0x0000aaaabbefbf80 in os_host_main_loop_wait (timeout=-1)
      at util/main-loop.c:233
  #4  main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:497
  #5  0x0000aaaabbe2aa30 in convert_do_copy (s=0xffffc123bb58) at 
  #6  img_convert (argc=<optimized out>, argv=<optimized out>) at 
  #7  0x0000aaaabbe2333c in main (argc=7, argv=<optimized out>) at 

  Reproduced w/ latest QEMU git (@ 53744e0a182)

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