[RFC,0/2] pidfs: handle multi-threaded exec and premature thread-group leader exit

Message

Christian Brauner March 17, 2025, 3 p.m. UTC

Oleg,

Don't kill me but this is another attempt at trying to make pidfd
polling for multi-threaded exec and premature thread-group leader exit
consistent.

A quick recap of these two cases:

(1) During a multi-threaded exec by a subthread, i.e., non-thread-group
    leader thread, all other threads in the thread-group including the
    thread-group leader are killed and the struct pid of the
    thread-group leader will be taken over by the subthread that called
    exec. IOW, two tasks change their TIDs.

(2) A premature thread-group leader exit means that the thread-group
    leader exited before all of the other subthreads in the thread-group
    have exited.

Both cases lead to inconsistencies for pidfd polling with PIDFD_THREAD.
Any caller that holds a PIDFD_THREAD pidfd to the current thread-group
leader may or may not see an exit notification on the file descriptor
depending on when poll is performed. If the poll is performed before the
exec of the subthread has concluded an exit notification is generated
for the old thread-group leader. If the poll is performed after the exec
of the subthread has concluded no exit notification is generated for the
old thread-group leader.

The correct behavior would be to simply not generate an exit
notification on the struct pid of a subhthread exec because the struct
pid is taken over by the subthread and thus remains alive.

But this is difficult to handle because a thread-group may exit
premature as mentioned in (2). In that case an exit notification is
reliably generated but the subthreads may continue to run for an
indeterminate amount of time and thus also may exec at some point.

So far there was no way to distinguish between (1) and (2) internally.
This tiny series tries to address this problem by remembering a
premature leader exit in struct pid and forgetting it when a subthread
execs and takes over the old thread-group leaders struct pid.

If that works correctly then no exit notifications are generated for a
PIDFD_THREAD pidfd for a thread-group leader until all subthreads have
been reaped. If a subthread should exec before no exit notification will
be generated until that task exits or it creates subthreads and repeates
the cycle.

Signed-off-by: Christian Brauner <[e-mail]>
---
Christian Brauner (2):
      pidfs: improve multi-threaded exec and premature thread-group leader exit polling
      selftests/pidfd: test multi-threaded exec polling

 fs/pidfs.c                                      | 28 +++++++++++++++++++++++--
 include/linux/pid.h                             |  3 ++-
 kernel/exit.c                                   | 24 +++++++++++++++++++--
 kernel/pid.c                                    |  9 ++++++++
 tools/testing/selftests/pidfd/pidfd_info_test.c | 23 ++++++++++----------
 5 files changed, 71 insertions(+), 16 deletions(-)
---
base-commit: c0ff2d6e30f20fd943eac5cdc3b0e89f2f2566ce
change-id: 20250317-work-pidfs-thread_group-141682f9a50a