Messages in this thread | | | From | "Michael Kerrisk (man-pages)" <> | Date | Thu, 10 Jan 2013 23:48:39 +0100 | Subject | Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision |
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Hi Lennart,
Ping!
Also, I'll add you to CC on Shawn's mail. Perhaps you can review/improve his patch. Or please send me an alternative.
Thanks,
Michael
On Tue, Apr 24, 2012 at 12:36 AM, Michael Kerrisk <mtk.manpages@gmail.com> wrote: > Lennart, > > On Sun, Jan 8, 2012 at 4:56 AM, Kay Sievers <kay.sievers@vrfy.org> wrote: >> Resending this, it got lost last year's September. >> >> We still need it to properly implement init-like service managers. >> >> Andrew, care to pick this up again? The issues raised the last year are >> all expected to be fixed. >> >> Thanks, >> Kay >> >> >> From: Lennart Poettering <lennart@poettering.net> >> Subject: prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision > > You have often enthusiastically pointed out to me pieces that are > missing from the man pages. When will you be sending me a man-pages > patch to document these new prctl() options that you've added? > > Thanks, > > Michael > > > >> Userspace service managers/supervisors need to track their started >> services. Many services daemonize by double-forking and get implicitly >> re-parented to PID 1. The service 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 happened. >> >> 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: >> >> before: >> # ps afx >> 253 ? Ss 0:00 /bin/dbus-daemon --system --nofork >> 294 ? Sl 0:00 /usr/libexec/polkit-1/polkitd >> 328 ? S 0:00 /usr/sbin/modem-manager >> 608 ? Sl 0:00 /usr/libexec/colord >> 658 ? Sl 0:00 /usr/libexec/upowerd >> 819 ? Sl 0:00 /usr/libexec/imsettings-daemon >> 916 ? Sl 0:00 /usr/libexec/udisks-daemon >> 917 ? S 0:00 \_ udisks-daemon: not polling any devices >> >> after: >> # ps afx >> 294 ? Ss 0:00 /bin/dbus-daemon --system --nofork >> 426 ? Sl 0:00 \_ /usr/libexec/polkit-1/polkitd >> 449 ? S 0:00 \_ /usr/sbin/modem-manager >> 635 ? Sl 0:00 \_ /usr/libexec/colord >> 705 ? Sl 0:00 \_ /usr/libexec/upowerd >> 959 ? Sl 0:00 \_ /usr/libexec/udisks-daemon >> 960 ? S 0:00 | \_ udisks-daemon: not polling any devices >> 977 ? Sl 0:00 \_ /usr/libexec/packagekitd >> >> This prctl is orthogonal to PID namespaces. PID namespaces are isolated >> from each other, while a service management process usually requires >> the services 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-demand. Both need init-like capabilities >> to be able to properly keep track of the services they start. >> >> Many thanks to Oleg for several rounds of review and insights. >> >> Reviewed-by: Oleg Nesterov <oleg@redhat.com> >> Signed-off-by: Lennart Poettering <lennart@poettering.net> >> Signed-off-by: Kay Sievers <kay.sievers@vrfy.org> >> --- >> >> include/linux/prctl.h | 3 +++ >> include/linux/sched.h | 12 ++++++++++++ >> kernel/exit.c | 28 +++++++++++++++++++++++----- >> kernel/fork.c | 3 +++ >> kernel/sys.c | 8 ++++++++ >> 5 files changed, 49 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 >> @@ -552,6 +552,18 @@ 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 >> @@ -687,11 +687,12 @@ 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) >> @@ -722,6 +723,23 @@ 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; >> + reaper != &init_task; >> + reaper = reaper->real_parent) { >> + if (same_thread_group(reaper, pid_ns->child_reaper)) >> + break; >> + 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 >> @@ -979,6 +979,9 @@ 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 >> @@ -1841,6 +1841,14 @@ 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; >> >> >> -- >> 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/ > > > > -- > Michael Kerrisk Linux man-pages maintainer; > http://www.kernel.org/doc/man-pages/ > Author of "The Linux Programming Interface", http://blog.man7.org/
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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