From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754959Ab1HSOVA (ORCPT ); Fri, 19 Aug 2011 10:21:00 -0400 Received: from mail-fx0-f46.google.com ([209.85.161.46]:42096 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752184Ab1HSOU5 (ORCPT ); Fri, 19 Aug 2011 10:20:57 -0400 Subject: Re: + prctl-add-pr_setget_child_reaper-to-allow-simple-process-supervision .patch added to -mm tree From: Kay Sievers To: Oleg Nesterov Cc: Lennart Poettering , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-man@vger.kernel.org, roland@hack.frob.com, torvalds@linux-foundation.org Date: Fri, 19 Aug 2011 16:20:51 +0200 In-Reply-To: <20110819131339.GB8411@redhat.com> References: <20110817115543.GA8745@redhat.com> <20110817134516.GA14136@redhat.com> <20110818124353.GA2839@tango.0pointer.de> <20110818142508.GA30959@redhat.com> <1313691091.1107.9.camel@mop> <20110818184857.GA12094@redhat.com> <1313717521.991.4.camel@mop> <20110819122503.GA8411@redhat.com> <20110819131339.GB8411@redhat.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.1.4 (3.1.4-1.fc16) Content-Transfer-Encoding: 7bit Message-ID: <1313763653.1119.12.camel@mop> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2011-08-19 at 15:13 +0200, Oleg Nesterov wrote: > On 08/19, Kay Sievers wrote: > But this has other (OK, minor) problems too, afaics. First of all, this > ->has_child_subreaper = T is not right when the caller exits. We should > not look for ->is_child_subreaper parent, our children should to find us. > > Right? The flags are still inherited. If the subreaper isn't there anymore, we will look it up and not find it. I think that's ok for the very unusual case that a service manager goes away and its children still run. We will just fall back to the usual PID 1 logic. > And. afaics this makes the semantics of prctl(REAPER) a bit unclear... > Suppose a task P does > > C1 = fork(); > > prctl(REAPER); > > C2 = fork(); > > In this case it "owns" the children of C2, but not C1. This is fine, and > perhaps this is even better. Yeah, thought about that too. I think it is what we want. > But what if P->parent did prctl(REAPER) too? Then P becomes the sub-reaper > for the tasks which were forked before prctl(). > > In short, in general the caller of prctl(REAPER) doesn't know how this > can affect the forks in the past. > > Again, again, I am not arguing. Just I think we should discuss everything > if we are going to add the new feature. I agree. I'll add a few lines of comment to explain this when we have a version that looks fine. I'll need to update the man page for the prctl anyway. It's probably the best place to explain the use of this flag. > Finally. I am not sure this is really better, but it seems we can > can ->has_child_subreape "more correct" with the same effect. > me->has_child_subreaper = > parent->has_child_subreaper || > parent->is_child_subreaper; Nice idea! Looks so much better, yes. Thanks a lot again, Kay version 4: - as we check from within 'flagged' processes only, we can avoid the &init_task or point-to-ourselves check - set parent->has_child_subreaper only in child processes so prctl(PR_SET_CHILD_SUBREAPER) toggle is cleaner regarding already existing and future childs version 3: - rename all child_reaper to child_subreaper to avoid confusion with the PID namespace child_reaper variable - check all possible threads of the reaper process for valid one - optimization: let processes inherit a flag to indicate that there is a subreaper to lookup, in case they need to be re-parented. version 2: - uses task->real_parent to walk up the chain of parents. - does not use init_task but the the parent pointer to itself - moves the flag into task->signal to have it process-wide and not per thread - moves the parent walk after the check for pid_ns->child_reaper == father - makes sure it does not return a PF_EXITING task - adds some explanation of SIGCHLD + wait() vs. async events like taskstats, to the changelog - updates the comments for find_new_reaper() From: Lennart Poettering Subject: prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision Userspace service managers/supervisors need to track their started services. Many services daemonize by double-forking and get implicitely re-parented to PID 1. The process manager will no longer be able to receive the SIGCHLD signals for them, and is no longer in charge of reaping the children with wait(). All information about the children is lost at the moment PID 1 cleans up the re-parented processes. With this prctl, a service manager process can mark itself as a sort of 'sub-init', able to stay as the parent for all orphaned processes created by the started services. All SIGCHLD signals will be delivered to the service manager. Receiving SIGCHLD and doing wait() is in cases of a service-manager much preferred over any possible asynchronous notification about specific PIDs, because the service manager has full access to the child process data in /proc and the PID can not be re-used until the wait(), the service-manager itself is in charge of, has happended. As a side effect, the relevant parent PID information does not get lost by a double-fork, which results in a more elaborate process tree and 'ps' output. This is orthogonal to PID namespaces. PID namespaces are isolated from each other, while a service management process usually requires the serices to live in the same namespace, to be able to talk to each other. Users of this will be the systemd per-user instance, which provides init-like functionality for the user's login session and D-Bus, which activates bus services on on-demand. Both will need init-like capabilities to be able to properly keep track of the services they start. Cc: Oleg Nesterov Signed-off-by: Lennart Poettering Signed-off-by: Kay Sievers --- include/linux/prctl.h | 3 +++ include/linux/sched.h | 12 ++++++++++++ kernel/exit.c | 26 +++++++++++++++++++++----- kernel/fork.c | 3 +++ kernel/sys.c | 8 ++++++++ 5 files changed, 47 insertions(+), 5 deletions(-) --- a/include/linux/prctl.h +++ b/include/linux/prctl.h @@ -102,4 +102,7 @@ #define PR_MCE_KILL_GET 34 +#define PR_SET_CHILD_SUBREAPER 35 +#define PR_GET_CHILD_SUBREAPER 36 + #endif /* _LINUX_PRCTL_H */ --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -550,6 +550,18 @@ struct signal_struct { int group_stop_count; unsigned int flags; /* see SIGNAL_* flags below */ + /* + * PR_SET_CHILD_SUBREAPER marks a process, like a service + * manager, to re-parent orphan (double-forking) child processes + * to this process instead of 'init'. The service manager is + * able to receive SIGCHLD signals and is able to investigate + * the process until it calls wait(). All children of this + * process will inherit a flag if they should look for a + * child_subreaper process at exit. + */ + unsigned int is_child_subreaper:1; + unsigned int has_child_subreaper:1; + /* POSIX.1b Interval Timers */ struct list_head posix_timers; --- a/kernel/exit.c +++ b/kernel/exit.c @@ -689,11 +689,12 @@ static void exit_mm(struct task_struct * } /* - * When we die, we re-parent all our children. - * Try to give them to another thread in our thread - * group, and if no such member exists, give it to - * the child reaper process (ie "init") in our pid - * space. + * When we die, we re-parent all our children, and try to: + * 1. give them to another thread in our thread group, if such a + * member exists + * 2. give it to the first anchestor process which prctl'd itself + * as a child_subreaper for its children (like a service manager) + * 3. give it to the init process (PID 1) in our pid namespace */ static struct task_struct *find_new_reaper(struct task_struct *father) __releases(&tasklist_lock) @@ -724,6 +725,21 @@ static struct task_struct *find_new_reap * forget_original_parent() must move them somewhere. */ pid_ns->child_reaper = init_pid_ns.child_reaper; + } else if (father->signal->has_child_subreaper) { + struct task_struct *reaper; + + /* find the first ancestor marked as child_subreaper */ + for (reaper = father->real_parent; + !same_thread_group(reaper, pid_ns->child_reaper); + reaper = reaper->real_parent) { + if (!reaper->signal->is_child_subreaper) + continue; + thread = reaper; + do { + if (!(thread->flags & PF_EXITING)) + return reaper; + } while_each_thread(reaper, thread); + } } return pid_ns->child_reaper; --- a/kernel/fork.c +++ b/kernel/fork.c @@ -987,6 +987,9 @@ static int copy_signal(unsigned long clo sig->oom_score_adj = current->signal->oom_score_adj; sig->oom_score_adj_min = current->signal->oom_score_adj_min; + sig->has_child_subreaper = current->signal->has_child_subreaper || + current->signal->is_child_subreaper; + mutex_init(&sig->cred_guard_mutex); return 0; --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1799,6 +1799,14 @@ SYSCALL_DEFINE5(prctl, int, option, unsi else error = PR_MCE_KILL_DEFAULT; break; + case PR_SET_CHILD_SUBREAPER: + me->signal->is_child_subreaper = !!arg2; + error = 0; + break; + case PR_GET_CHILD_SUBREAPER: + error = put_user(me->signal->is_child_subreaper, + (int __user *) arg2); + break; default: error = -EINVAL; break;