On 30/06/16 13:32, Alex Bennée wrote: > Sergey Fedorov <serge.f...@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?
I'm thinking of waiting for completion of safe works in cpu_exec_start() as well as in cpu_exec_end(). Regards, Sergey
