• Eric W. Biederman's avatar
    proc: Ensure we see the exit of each process tid exactly once · 6b03d130
    Eric W. Biederman authored
    When the thread group leader changes during exec and the old leaders
    thread is reaped proc_flush_pid will flush the dentries for the entire
    process because the leader still has it's original pid.
    
    Fix this by exchanging the pids in an rcu safe manner,
    and wrapping the code to do that up in a helper exchange_tids.
    
    When I removed switch_exec_pids and introduced this behavior
    in d73d6529 ("[PATCH] pidhash: kill switch_exec_pids") there
    really was nothing that cared as flushing happened with
    the cached dentry and de_thread flushed both of them on exec.
    
    This lack of fully exchanging pids became a problem a few months later
    when I introduced 48e6484d ("[PATCH] proc: Rewrite the proc dentry
    flush on exit optimization").  Which overlooked the de_thread case
    was no longer swapping pids, and I was looking up proc dentries
    by task->pid.
    
    The current behavior isn't properly a bug as everything in proc will
    continue to work correctly just a little bit less efficiently.  Fix
    this just so there are no little surprise corner cases waiting to bite
    people.
    
    -- Oleg points out this could be an issue in next_tgid in proc where
       has_group_leader_pid is called, and reording some of the assignments
       should fix that.
    
    -- Oleg points out this will break the 10 year old hack in __exit_signal.c
    >	/*
    >	 * This can only happen if the caller is de_thread().
    >	 * FIXME: this is the temporary hack, we should teach
    >	 * posix-cpu-timers to handle this case correctly.
    >	 */
    >	if (unlikely(has_group_leader_pid(tsk)))
    >		posix_cpu_timers_exit_group(tsk);
    
    The code in next_tgid has been changed to use PIDTYPE_TGID,
    and the posix cpu timers code has been fixed so it does not
    need the 10 year old hack, so this should be safe to merge
    now.
    
    Link: https://lore.kernel.org/lkml/87h7x3ajll.fsf_-_@x220.int.ebiederm.org/Acked-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
    Fixes: 48e6484d ("[PATCH] proc: Rewrite the proc dentry flush on exit optimization").
    Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
    6b03d130
pid.c 16.7 KB