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
