On Tue, Sep 19, 2017 at 04:48:35PM -0500, Eric Blake wrote: > 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.
Yeah, that's something I liked as well. > > [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] Good to learn it! Thanks for digging the whole thing up. > > > > > 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. If this patch is doable, I'll add explicit reason to commit message. Paolo/Markus, would any of you help confirm this change? (considering Paolo introduced commit d622cb587) One thing I slightly not sure of is that, some device realization has this code path (take fsl_imx25_realize() as example): fsl_imx25_realize qemu_chr_new qemu_chr_new_noreplay char is_mux? monitor_init (note: I never know why we create the monitor in chardev creation... would there be a better place?) Especially considering some integrated devices can be created along with machine init. Anyway, this patch was trying to cleanup the things a bit, and also more convenient for me to add new codes upon. If any of us think it's not safe enough, please say explicitly, and I can drop it and do the rest in "the ugly way". Thanks, -- Peter Xu