Re: [Qemu-devel] [RFC 07/15] monitor: unify global init

[Eric Blake]

On 09/19/2017 04:35 PM, Eric Blake wrote:
> On 09/14/2017 02:50 AM, Peter Xu wrote:
>> There are many places for monitor init its globals, at least:
>>

> Are we sure that this new function is called sooner than any access to
> monitor_lock,
> 
>> -static void __attribute__((constructor)) monitor_lock_init(void)
>> -{
>> -    qemu_mutex_init(&monitor_lock);
>> -}
> 
> especially since the old code initialized the lock REALLY early?

(Partially) answering myself:

> 
> Pre-patch, a breakpoint on main() and on monitor_lock_init() fires on
> monitor_lock_init() first, prior to main.
> 
> Breakpoint 2, monitor_lock_init () at /home/eblake/qemu/monitor.c:4089
> 4089      qemu_mutex_init(&monitor_lock);
> (gdb) c
> Continuing.
> [New Thread 0x7fffce225700 (LWP 26380)]
> 
> Thread 1 "qemu-system-x86" hit Breakpoint 1, main (argc=5,
>     argv=0x7fffffffdc88, envp=0x7fffffffdcb8) at vl.c:3077
> 3077  {

Also, pre-patch, 'watch monitor_lock.initialized' and 'watch
monitor_lock.lock.__data.__lock' show that the lock is first utilized at:

(gdb) bt
#0  0x00007fffdac59e12 in __GI___pthread_mutex_lock
(mutex=0x555556399340 <monitor_lock>) at ../nptl/pthread_mutex_lock.c:80
#1  0x0000555555ce01ed in qemu_mutex_lock (mutex=0x555556399340
<monitor_lock>)
    at util/qemu-thread-posix.c:65
#2  0x00005555557bc8b8 in monitor_init (chr=0x55555690bf70, flags=4)
    at /home/eblake/qemu/monitor.c:4126
#3  0x000055555591ae80 in mon_init_func (opaque=0x0,
opts=0x55555688e3d0, errp=0x0) at vl.c:2482
#4  0x0000555555cf3e63 in qemu_opts_foreach (list=0x555556225200
<qemu_mon_opts>, func=0x55555591ad33 <mon_init_func>, opaque=0x0, errp=0x0)
    at util/qemu-option.c:1104
#5  0x0000555555920128 in main (argc=5, argv=0x7fffffffdc88,
envp=0x7fffffffdcb8) at vl.c:4670

and double-checking qemu_mutex_lock, our .initialized member provides
NICE runtime checking that we don't use an uninitialized lock.  So the
fact that your patch doesn't assert means your later initialization is
still fine.

[TIL: the gdb 'watch' command is cool, but it's better if you watch only
4 or 8 bytes at a time; I first tried 'watch monitor_lock', but that's
hundreds of times slower as hardware can't watch that much data at once,
at which point gdb emulates it by single-stepping the entire program]

> 
> Post-patch, the mutex is not initialized until well after main().  So
> the real question is what (if anything) is using the lock in between
> those two points?

According to gdb watchpoints, no.

> 
> Hmm - it may be that we needed it back before commit 05875687, when we
> really did depend on MODULE_INIT_QAPI, but it is something we forgot to
> cleanup in the meantime?

So what I didn't debug was whether the constructor attribute was
mandatory in the past, and if so, which commit made it no longer
mandatory (my mention of commit 05875687 is only a guess).

> 
> If nothing else, the commit message should call out that dropping
> __attribute__((constructor)) nonsense is intentional (if it was indeed
> nonsense).
> 

This part is still true.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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