From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54600) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIYRM-0003zx-CU for qemu-devel@nongnu.org; Thu, 30 Jun 2016 05:39:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bIYRJ-0001kT-8H for qemu-devel@nongnu.org; Thu, 30 Jun 2016 05:39:20 -0400 Received: from mail-lf0-x243.google.com ([2a00:1450:4010:c07::243]:34800) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIYRI-0001kI-Vv for qemu-devel@nongnu.org; Thu, 30 Jun 2016 05:39:17 -0400 Received: by mail-lf0-x243.google.com with SMTP id l184so7783169lfl.1 for ; Thu, 30 Jun 2016 02:39:16 -0700 (PDT) References: <1466375313-7562-1-git-send-email-sergey.fedorov@linaro.org> <1466375313-7562-7-git-send-email-sergey.fedorov@linaro.org> <87lh1o0y1k.fsf@linaro.org> From: Sergey Fedorov Message-ID: <5774E8C2.1050506@gmail.com> Date: Thu, 30 Jun 2016 12:39:14 +0300 MIME-Version: 1.0 In-Reply-To: <87lh1o0y1k.fsf@linaro.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [RFC 6/8] linux-user: Support CPU work queue List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?UTF-8?Q?Alex_Benn=c3=a9e?= , Sergey Fedorov Cc: qemu-devel@nongnu.org, Riku Voipio , Peter Crosthwaite , patches@linaro.org, Paolo Bonzini , Richard Henderson 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 > > > > 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