Re: [Qemu-devel] [PULL 5/6] console: remove do_safe_dpy_refresh

[Laurent Vivier <lvivier@redhat.com>]

On 18/07/2017 15:07, Laurent Vivier wrote:
> On 21/06/2017 15:23, Gerd Hoffmann wrote:
>> Drop the temporary workaround for the broken display updates.
>> All display adapters are updated, so this should be safe without
>> causing regressions.
> 
> It seems it breaks QMP command 'migrate "exec:cat>mig"'.
> 
> The command hangs and doesn't create the file.
> 
> It happens with qemu-system-ppc64 on x86 (so TCG mode).
> 
> my command:
> 
>    ./ppc64-softmmu/qemu-system-ppc64 -serial mon:stdio
> 
> I wait SLOF fails to find an OS, and:
> 
>     Ctrl-a c
>     (qemu) migrate -d "exec:cat>mig"
> 
> The file is not created and the command hangs:
> 
> #0  in __lll_lock_wait
> #1  in pthread_mutex_lock
> #2  in qemu_mutex_lock
> #3  in rcu_init_lock
> #4  in fork
> #5  in qemu_fork
> #6  in qio_channel_command_new_spawn
> #7  in exec_start_outgoing_migration
> #8  in qmp_migrate
> ...
> 
> It looks like a deadlock.

I think this patch is not the cause of the problem, the one it removes
just unlocks the deadlock by playing with locks.

We have a rcu_init_lock() on fork() because of:

utils/rcu.c:

static void __attribute__((__constructor__)) rcu_init(void)
{
#ifdef CONFIG_POSIX
    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_unlock);
#endif
    rcu_init_complete();
}

The QMP thread hangs on:

(gdb) p rcu_sync_lock
$1 = {lock = {__data = {__lock = 2, __count = 0, __owner = 23865,
      __nusers = 1, __kind = 0, __spins = 0, __elision = 0, __list = {
        __prev = 0x0, __next = 0x0}},
    __size = "\002\000\000\000\000\000\000\000\071]\000\000\001", '\000'
<repeats 26 times>, __align = 2}, initialized = true}


The lock is already taken by thread 2:

(gdb) info thread
  Id   Target Id         Frame
  1    Thread 0x7f1cf02fdf00 (LWP 23864) "qemu-system-ppc"
0x00007f1cd914b37d in __lll_lock_wait () from /lib64/libpthread.so.0
* 2    Thread 0x7f1cc9762700 (LWP 23865) "qemu-system-ppc"
0x00007f1cd410daa9 in syscall () from /lib64/libc.so.6
  3    Thread 0x7f1cbf8d5700 (LWP 23866) "qemu-system-ppc"
0x00007f1cd914b37d in __lll_lock_wait () from /lib64/libpthread.so.0

(gdb) bt
#0  0x00007f1cd410daa9 in syscall () at /lib64/libc.so.6
#1  0x000055ab028ddda2 in qemu_futex_wait
#2  0x000055ab028ddda2 in qemu_event_wait
#3  0x000055ab028eda2b in wait_for_readers
#4  0x000055ab028eda2b in synchronize_rcu
#5  0x000055ab028edc5b in call_rcu_thread
#6  0x00007f1cd914273a in start_thread ()
#7  0x00007f1cd4113e0f in clone ()

So it seems we cannot fork() from QMP?
[cc: Paolo]

Any comments?

Laurent