Re: [Qemu-devel] "iothread: release iothread around aio_poll" causes random hangs at startup

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 09.06.2015 um 04:28 schrieb Fam Zheng:
> On Tue, 06/02 16:36, Christian Borntraeger wrote:
>> Paolo,
>>
>> I bisected 
>> commit a0710f7995f914e3044e5899bd8ff6c43c62f916
>> Author:     Paolo Bonzini <pbonzini@redhat.com>
>> AuthorDate: Fri Feb 20 17:26:52 2015 +0100
>> Commit:     Kevin Wolf <kwolf@redhat.com>
>> CommitDate: Tue Apr 28 15:36:08 2015 +0200
>>
>>     iothread: release iothread around aio_poll
>>
>> to cause a problem with hanging guests.
>>
>> Having many guests all with a kernel/ramdisk (via -kernel) and
>> several null block devices will result in hangs. All hanging 
>> guests are in partition detection code waiting for an I/O to return
>> so very early maybe even the first I/O.
>>
>> Reverting that commit "fixes" the hangs.
>> Any ideas?
> 
> Christian, I can't reproduce this on my x86 box with virtio-blk-pci. Do you
> have a reproducer for x86? Or could you collect backtraces for all the threads
> in QEMU when it hangs?
> 
> My long shot is that the main loop is blocked at aio_context_acquire(ctx),
> while the iothread of that ctx is blocked at aio_poll(ctx, blocking).

Here is a backtrace on s390. I need 2 or more disks, (one is not enough).

Thread 5 (Thread 0x3fffb406910 (LWP 74602)):
#0  0x000003fffc0bde8e in syscall () from /lib64/libc.so.6
#1  0x00000000801dd282 in futex_wait (val=4294967295, ev=0x8079c6c4 <rcu_call_ready_event>) at /home/cborntra/REPOS/qemu/util/qemu-thread-posix.c:301
#2  qemu_event_wait (ev=ev@entry=0x8079c6c4 <rcu_call_ready_event>) at /home/cborntra/REPOS/qemu/util/qemu-thread-posix.c:399
#3  0x00000000801ec75c in call_rcu_thread (opaque=<optimized out>) at /home/cborntra/REPOS/qemu/util/rcu.c:233
#4  0x000003fffc16f4e6 in start_thread () from /lib64/libpthread.so.0
#5  0x000003fffc0c30fa in thread_start () from /lib64/libc.so.6

Thread 4 (Thread 0x3fffabf7910 (LWP 74604)):
#0  0x000003fffc0b75d6 in ppoll () from /lib64/libc.so.6
#1  0x000000008016fbd0 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:77
#2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=timeout@entry=-1) at /home/cborntra/REPOS/qemu/qemu-timer.c:310
#3  0x0000000080170d32 in aio_poll (ctx=0x807d6a70, blocking=blocking@entry=true) at /home/cborntra/REPOS/qemu/aio-posix.c:274
#4  0x00000000800b3758 in iothread_run (opaque=0x807d6690) at /home/cborntra/REPOS/qemu/iothread.c:41
#5  0x000003fffc16f4e6 in start_thread () from /lib64/libpthread.so.0
#6  0x000003fffc0c30fa in thread_start () from /lib64/libc.so.6

Thread 3 (Thread 0x3fffa3f7910 (LWP 74605)):
#0  0x000003fffc0b75d6 in ppoll () from /lib64/libc.so.6
#1  0x000000008016fbd0 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:77
#2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=timeout@entry=-1) at /home/cborntra/REPOS/qemu/qemu-timer.c:310
#3  0x0000000080170d32 in aio_poll (ctx=0x807d94a0, blocking=blocking@entry=true) at /home/cborntra/REPOS/qemu/aio-posix.c:274
#4  0x00000000800b3758 in iothread_run (opaque=0x807d6f60) at /home/cborntra/REPOS/qemu/iothread.c:41
#5  0x000003fffc16f4e6 in start_thread () from /lib64/libpthread.so.0
#6  0x000003fffc0c30fa in thread_start () from /lib64/libc.so.6

Thread 2 (Thread 0x3fff8a21910 (LWP 74625)):
#0  0x000003fffc0b90a2 in ioctl () from /lib64/libc.so.6
#1  0x0000000080056a46 in kvm_vcpu_ioctl (cpu=cpu@entry=0x81f3b620, type=type@entry=44672) at /home/cborntra/REPOS/qemu/kvm-all.c:1916
#2  0x0000000080056b08 in kvm_cpu_exec (cpu=cpu@entry=0x81f3b620) at /home/cborntra/REPOS/qemu/kvm-all.c:1775
#3  0x00000000800445de in qemu_kvm_cpu_thread_fn (arg=0x81f3b620) at /home/cborntra/REPOS/qemu/cpus.c:979
#4  0x000003fffc16f4e6 in start_thread () from /lib64/libpthread.so.0
#5  0x000003fffc0c30fa in thread_start () from /lib64/libc.so.6

Thread 1 (Thread 0x3fffb408bc0 (LWP 74580)):
#0  0x000003fffc0b75d6 in ppoll () from /lib64/libc.so.6
#1  0x000000008016fbb0 in ppoll (__ss=0x0, __timeout=0x3ffffd64438, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:77
#2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=timeout@entry=999000000) at /home/cborntra/REPOS/qemu/qemu-timer.c:322
#3  0x000000008016f230 in os_host_main_loop_wait (timeout=999000000) at /home/cborntra/REPOS/qemu/main-loop.c:239
#4  main_loop_wait (nonblocking=<optimized out>) at /home/cborntra/REPOS/qemu/main-loop.c:494
#5  0x000000008001346a in main_loop () at /home/cborntra/REPOS/qemu/vl.c:1789
#6  main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at /home/cborntra/REPOS/qemu/vl.c:4391