[Qemu-devel] [Bug 1817865] Re: deadlock in prepare_mmio_access reading mmio

[Vic]

In order to fix the issue I think we need a call like
"qemu_mutex_trylock_iothread()" in order to combine
qemu_mutex_iothread_locked() and qemu_mutex_lock_iothread() into a
single atomic operation. Any comments?

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https://bugs.launchpad.net/bugs/1817865

Title:
  deadlock in prepare_mmio_access reading mmio

Status in QEMU:
  New

Bug description:
  QEMU release 3.1.0; Guest running win10.

  The deadlock happens in prepare_mmio_access call when the guest is
  trying to read MMIO address 0xFEBC00C0. This address belongs to
  "Intel(R) PRO/1000 MT Network Connection" as shown in Windows 10
  device manager. A quick analysis has shown that in prepare_mmio_access
  call:

  static bool prepare_mmio_access(MemoryRegion *mr)
  {
      bool unlocked = !qemu_mutex_iothread_locked();
      bool release_lock = false;

      if (unlocked && mr->global_locking) {
          qemu_mutex_lock_iothread();
  ...

  "qemu_mutex_iothread_locked()" and "qemu_mutex_lock_iothread()" are
  not atomic operation, so the global mutex could become locked by "util
  /main-loop.c" (line = 236) after "qemu_mutex_iothread_locked()" is
  called.

  GDB backtrace analysis as following:

  (gdb) thread 9
  [Switching to thread 9 (Thread 0x7fffe6b7c700 (LWP 14405))]
  #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:103
  103   ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file or 
directory.
  (gdb) bt
  #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:103
  #1  0x00007ffff748c714 in __GI___pthread_mutex_lock (mutex=0x555556518340 
<qemu_global_mutex>) at ../nptl/pthread_mutex_lock.c:80
  #2  0x0000555555d8ae7b in qemu_mutex_lock_impl (mutex=0x555556518340 
<qemu_global_mutex>, 
      file=0x555555e7a298 "/home/vic/Projects/tc-qemu/exec.c", line=3197) at 
util/qemu-thread-posix.c:66
  #3  0x000055555584cf66 in qemu_mutex_lock_iothread_impl (file=0x555555e7a298 
"/home/vic/Projects/tc-qemu/exec.c", line=3197)
      at /home/vic/Projects/tc-qemu/cpus.c:1849
  #4  0x0000555555804ccc in prepare_mmio_access (mr=0x5555575b9000) at 
/home/vic/Projects/tc-qemu/exec.c:3197
  #5  0x0000555555804f90 in flatview_read_continue (fv=0x7fffd86f2390, 
addr=4273733824, attrs=..., buf=0x7ffff7fcc028 "ۜ/o", len=4, 
      addr1=192, l=4, mr=0x5555575b9000) at 
/home/vic/Projects/tc-qemu/exec.c:3292
  #6  0x0000555555805136 in flatview_read (fv=0x7fffd86f2390, addr=4273733824, 
attrs=..., buf=0x7ffff7fcc028 "ۜ/o", len=4)
      at /home/vic/Projects/tc-qemu/exec.c:3332
  #7  0x00005555558051aa in address_space_read_full (as=0x555556517bc0 
<address_space_memory>, addr=4273733824, attrs=..., 
      buf=0x7ffff7fcc028 "ۜ/o", len=4) at /home/vic/Projects/tc-qemu/exec.c:3345
  #8  0x0000555555805281 in address_space_rw (as=0x555556517bc0 
<address_space_memory>, addr=4273733824, attrs=..., 
      buf=0x7ffff7fcc028 "ۜ/o", len=4, is_write=false) at 
/home/vic/Projects/tc-qemu/exec.c:3375
  #9  0x0000555555886199 in kvm_cpu_exec (cpu=0x5555566d1800) at 
/home/vic/Projects/tc-qemu/accel/kvm/kvm-all.c:2031
  #10 0x000055555584c001 in qemu_kvm_cpu_thread_fn (arg=0x5555566d1800) at 
/home/vic/Projects/tc-qemu/cpus.c:1281
  #11 0x0000555555d8b9f7 in qemu_thread_start (args=0x5555566f3c00) at 
util/qemu-thread-posix.c:498
  #12 0x00007ffff7489fa3 in start_thread (arg=<optimized out>) at 
pthread_create.c:486
  #13 0x00007ffff73ba80f in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:95
  (gdb) f 2
  #2  0x0000555555d8ae7b in qemu_mutex_lock_impl (mutex=0x555556518340 
<qemu_global_mutex>, 
      file=0x555555e7a298 "/home/vic/Projects/tc-qemu/exec.c", line=3197) at 
util/qemu-thread-posix.c:66
  66        err = pthread_mutex_lock(&mutex->lock);
  (gdb) p mutex
  $1 = (QemuMutex *) 0x555556518340 <qemu_global_mutex>
  (gdb) p *mutex
  $2 = {lock = pthread_mutex_t = {Type = Normal, Status = Acquired, possibly 
with waiters, Owner ID = 14393, Robust = No, Shared = No, 
      Protocol = None}, file = 0x555555fc2a64 "util/main-loop.c", line = 236, 
initialized = true}
  (gdb) thread 1
  [Switching to thread 1 (Thread 0x7ffff495cc00 (LWP 14393))]
  #0  0x00007ffff7493832 in __libc_recvfrom (fd=147, buf=0x7fffd81ce0cc, 
len=1500, flags=0, addr=..., addrlen=0x7fffffffd9a8)
      at ../sysdeps/unix/sysv/linux/recvfrom.c:27
  27    ../sysdeps/unix/sysv/linux/recvfrom.c: No such file or directory.
  (gdb) bt
  #0  0x00007ffff7493832 in __libc_recvfrom (fd=147, buf=0x7fffd81ce0cc, 
len=1500, flags=0, addr=..., addrlen=0x7fffffffd9a8)
      at ../sysdeps/unix/sysv/linux/recvfrom.c:27
  #1  0x0000555555c18bf3 in sorecvfrom (so=0x7fffd818fe60) at slirp/socket.c:584
  #2  0x0000555555c146f9 in slirp_pollfds_poll (pollfds=0x555556655470, 
select_error=0) at slirp/slirp.c:753
  #3  0x0000555555d8675b in main_loop_wait (nonblocking=0) at 
util/main-loop.c:498
  #4  0x00005555559d2194 in main_loop () at vl.c:1893
  #5  0x00005555559d9d93 in main (argc=16, argv=0x7fffffffdff8, 
envp=0x7fffffffe080) at vl.c:4692

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