Re: [Qemu-devel] [RFC 6/8] linux-user: Support CPU work queue

[Sergey Fedorov]

On 29/06/16 19:17, Alex Bennée wrote:
> So I think there is a deadlock we can get with the async work:
>
> (gdb) thread apply all bt
>
> Thread 11 (Thread 0x7ffefeca7700 (LWP 2912)):
> #0  pthread_cond_wait@@GLIBC_2.3.2 () at 
> ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
> #1  0x00005555555cb777 in wait_cpu_work () at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/main.c:155
> #2  0x00005555555a0cee in wait_safe_cpu_work () at 
> /home/alex/lsrc/qemu/qemu.git/cpu-exec-common.c:87
> #3  0x00005555555cb8fe in cpu_exec_end (cpu=0x555555bb67e0) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/main.c:222
> #4  0x00005555555cc7a7 in cpu_loop (env=0x555555bbea58) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/main.c:749
> #5  0x00005555555db0b2 in clone_func (arg=0x7fffffffc9c0) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/syscall.c:5424
> #6  0x00007ffff6bed6fa in start_thread (arg=0x7ffefeca7700) at 
> pthread_create.c:333
> #7  0x00007ffff6923b5d in clone () at 
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
>
> <a bunch of other threads doing the same and then...>
>
> Thread 3 (Thread 0x7ffff7f38700 (LWP 2904)):
> #0  0x00005555555faf5d in safe_syscall_base ()
> #1  0x00005555555cfeaf in safe_futex (uaddr=0x7ffff528a0a4, op=128, val=1, 
> timeout=0x0, uaddr2=0x0, val3=-162668384)
>     at /home/alex/lsrc/qemu/qemu.git/linux-user/syscall.c:706
> #2  0x00005555555dd7cc in do_futex (uaddr=4132298916, op=128, val=1, 
> timeout=0, uaddr2=0, val3=-162668384)
>     at /home/alex/lsrc/qemu/qemu.git/linux-user/syscall.c:6246
> #3  0x00005555555e8cdb in do_syscall (cpu_env=0x555555a81118, num=240, 
> arg1=-162668380, arg2=128, arg3=1, arg4=0, arg5=0, arg6=-162668384,
>     arg7=0, arg8=0) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/syscall.c:10642
> #4  0x00005555555cd20e in cpu_loop (env=0x555555a81118) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/main.c:883
> #5  0x00005555555db0b2 in clone_func (arg=0x7fffffffc9c0) at 
> /home/alex/lsrc/qemu/qemu.git/linux-user/syscall.c:5424
> #6  0x00007ffff6bed6fa in start_thread (arg=0x7ffff7f38700) at 
> pthread_create.c:333
> #7  0x00007ffff6923b5d in clone () at 
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
>
> So everything is stalled awaiting this thread waking up and draining
> its queue. So for linux-user I think we need some mechanism to kick
> these syscalls which I assume means throwing a signal at it.

Nice catch! How did you get it? We always go through cpu_exec_end()
before serving a guest syscall and always go through cpu_exec_start()
before entering the guest code execution loop. If we always schedule
safe work on the current thread's queue then I think there's a way to
make it safe and avoid kicking syscalls.

Thanks,
Sergey