On Thu, Aug 04, 2016 at 08:46:03AM +1000, Michael Kerrisk (man-pages) wrote: > [Adding a few people to CC, who may also be interested] > > Hi Jann, > > On 08/02/2016 10:25 AM, Jann Horn wrote: > >Document the /proc/[pid]/task/[tid]/children interface from CRIU, and more > >importantly, document why it's usually not a good interface. > >--- > > man5/proc.5 | 23 +++++++++++++++++++++-- > > 1 file changed, 21 insertions(+), 2 deletions(-) > > > >diff --git a/man5/proc.5 b/man5/proc.5 > >index 0970c72..ddb14cc 100644 > >--- a/man5/proc.5 > >+++ b/man5/proc.5 > >@@ -2325,14 +2325,33 @@ the corresponding files under > > .I task/[tid] > > may have different values (e.g., various fields in each of the > > .I task/[tid]/status > >-files may be different for each thread). > >- > >+files may be different for each thread), > >+.\" in particular: "children" :/ > >+or they might not exist in > >+.I /proc/[pid] > >+at all. > > .\" The following was still true as at kernel 2.6.13 > > In a multithreaded process, the contents of the > > .I /proc/[pid]/task > > directory are not available if the main thread has already terminated > > (typically by calling > > .BR pthread_exit (3)). > >+ > >+.TP > >+.IR /proc/[pid]/task/[tid]/children " (since Linux 3.5)" > >+.\" commit 818411616baf46ceba0cff6f05af3a9b294734f7 > >+A space-separated list of child tasks of this task. > >+Each child task is represented by its TID. > >+ > >+.\" see comments in get_children_pid() in fs/proc/array.c > >+This does not work properly if children of the target task exit while > >+the file is being read! > >+Exiting children may cause non-exiting children to be omitted from > >+the list. > >+This makes this interface even more unreliable than classic PID-based > >+approaches if the inspected task and its children aren't frozen, and > >+most code should probably not use this interface. > >+ > > .TP > > .IR /proc/[pid]/timers " (since Linux 3.10)" > > .\" commit 5ed67f05f66c41e39880a6d61358438a25f9fee5 > > Thanks for this! I tweaked your text somewhat, and added some > details about kernel configuration options, so that now the text > reads: > > /proc/[pid]/task/[tid]/children (since Linux 3.5) > A space-separated list of child tasks of this task. > Each child task is represented by its TID. > > This option is intended for use by the checkpoint- > restore (CRIU) system, and reliably provides a list of > children only if all of the child processes are > stopped or frozen. It does not work properly if chil‐ > dren of the target task exit while the file is being > read! Exiting children may cause non-exiting children > to be omitted from the list. This makes this inter‐ > face even more unreliable than classic PID-based > approaches if the inspected task and its children > aren't frozen, and most code should probably not use > this interface. > > Until Linux 4.2, the presence of this file was gov‐ > erned by the CONFIG_CHECKPOINT_RESTORE kernel configu‐ > ration option. Since Linux 4.2, it it is governed by > the CONFIG_PROC_CHILDREN option. > > Look okay? Looks good to me.
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