[Qemu-devel] [Bug 1805256] Re: qemu-img hangs on high core count ARM system

[贞贵李 <1805256@bugs.launchpad.net>]

I can reproduce this problem with qemu.git/matser. It still exists in qemu.git/matser. I found that when an IO return in worker threads and want to call aio_notify to wake up main_loop, but it found that ctx->notify_me is cleared to 0 by main_loop in aio_ctx_check by calling atomic_and(&ctx->notify_me, ~1) . So worker thread won't write enventfd to notify main_loop.If such a scene happens, the main_loop will hang:
    main loop                        worker thread1                  worker thread2
-----------------------------------------------------------------------------------------------
    qemu_poll_ns                    aio_worker
                            qemu_bh_schedule(pool->completion_bh)
    glib_pollfds_poll
    g_main_context_check
    aio_ctx_check
    atomic_and(&ctx->notify_me, ~1)                                aio_worker
                                                      qemu_bh_schedule(pool->completion_bh)
    /* do something for event */
    qemu_poll_ns
    /* hangs !!!*/

As we known, ctx->notify_me will be visited by worker thread and main
loop. I thank we should add a lock protection for ctx->notify_me to
avoid this happend.what do you thank so?

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https://bugs.launchpad.net/bugs/1805256

Title:
  qemu-img hangs on high core count ARM system

Status in QEMU:
  Confirmed

Bug description:
  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, nfds=187650274213760, 
      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 out>, 
      __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 qemu-img.c:1980
  #6  img_convert (argc=<optimized out>, argv=<optimized out>) at qemu-img.c:2456
  #7  0x0000aaaabbe2333c in main (argc=7, argv=<optimized out>) at qemu-img.c:4975

  Reproduced w/ latest QEMU git (@ 53744e0a182)

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