Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759148AbaDJTiW (ORCPT ); Thu, 10 Apr 2014 15:38:22 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53030 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759097AbaDJTiV (ORCPT ); Thu, 10 Apr 2014 15:38:21 -0400 Date: Thu, 10 Apr 2014 21:38:20 +0200 From: Oleg Nesterov To: Steven Rostedt Cc: Mathieu Desnoyers , Frederic Weisbecker , LKML , Andrew Morton , Ingo Molnar , Hendrik Brueckner Subject: Re: [PATCH RESEND 2/2] tracing: syscall_regfunc() should not skip kernel threads Message-ID: <20140410193820.GA15624@redhat.com> References: <20140409170505.GA27638@redhat.com> <20140409170616.GC27638@redhat.com> <20140410092842.1f9a8760@gandalf.local.home> <20140410133855.GC12228@redhat.com> <20140410102816.24337ffe@gandalf.local.home> <20140410144655.GA25316@redhat.com> <20140410110848.64c3f25e@gandalf.local.home> <20140410175705.GB32332@redhat.com> <20140410181417.GA2104@redhat.com> <20140410151340.7d24aafd@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140410151340.7d24aafd@gandalf.local.home> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/10, Steven Rostedt wrote: > > On Thu, 10 Apr 2014 20:14:17 +0200 > Oleg Nesterov wrote: > > > > And I forgot to mention, given that the kernel_thread() callback should > > call do_exit() itself, then this part of cc3b13c11c567c69a63 > > > > one case when a kernel thread can reach the > > usual syscall exit tracing path: when we create a kernel thread, the > > child comes to ret_from_fork > > > > is no longer relevant? A PF_KTHREAD child should never return from the > > callback and thus it should never do "jmp syscall_exit" ? > > > > Are you sure. Not. > On set up of the kthread, create_kthread() calls > kernel_thread() with "kthread()" as its first parameter. > > kernel_thread() then calls do_fork() passing the "kthread" function as > the stack_start parameter, which if you follow where that goes, it gets > to copy_thread() in process_[63][42].c which assigns sp (the function) > to the bx register for the PF_KTHREAD case. But more importantly, it > sets up the stack to have ip pointing to ret_from_kernel_thread (32 bit > version). > > The jmp syscall_exit when it goes to return to "userspace" will in > actuality return to ret_from_kernel_thread (32 bit). Which this does: > > call *PT_EBX(%esp) > > which calls your handler. But then again, this calls syscall_exit when > done, which probably will never be hit as kthread() calls do_exit() > itself. Perhaps if something goes wrong, syscall_exit can handle any > faults that can happen? > > For 64 bit, the check for kernel thread is in ret_from_fork itself. > which does the call *%rbx, but again, if it fails, it then calls > int_ret_from_sys_call, which it may also handle faults. See my previous email. I _think_ that the kernel thread can only return from "call *%rbx" if it is no longer a kernel thread, iow, do_execve() was called. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/