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

["Alex Bennée" <alex.bennee@linaro.org>]

Sergey Fedorov <serge.fdrv@gmail.com> writes:

> 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?

Running pigz (armhf, debian) to compress stuff.

> 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.

Not let the signals complete until safe work is done?
>
> Thanks,
> Sergey


--
Alex Bennée