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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Alexey Gladkov <legion@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>
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Date:   Tue, 28 Apr 2020 07:16:44 -0500
In-Reply-To: <CAHk-=wjSM9mgsDuX=ZTy2L+S7wGrxZMcBn054As_Jyv8FQvcvQ@mail.gmail.com>
        (Linus Torvalds's message of "Mon, 27 Apr 2020 13:27:40 -0700")
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Subject: [PATCH v4 0/2] proc: Ensure we see the exit of each process tid exactly
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In the work to remove proc_mnt I noticed that we were calling
proc_flush_task now proc_flush_pid possibly multiple times for the same
pid because of how de_thread works.

This is a bare minimal patchset to sort out de_thread, by introducing
exchange_tids and the helper of exchange_tids hlists_swap_heads_rcu.

The actual call of exchange_tids should be slowpath so I have
prioritized readability over getting every last drop of performance.

I have also read through a bunch of the code to see if I could find
anything that would be affected by this change.  Users of
has_group_leader_pid were a good canidates.  But I also looked at other
cases that might have a pid->task->pid transition.  I ignored other
sources of races with de_thread and exec as those are preexisting.

I found a close call with send_signals user of task_active_pid_ns, but
all pids of a thread group are guaranteeds to be in the same pid
namespace so there is not a problem.

I found a few pieces of debugging code that do:

	task = pid_task(pid, PIDTYPE_PID);
        if (task) {
        	printk("%u\n", task->pid);
        }

But I can't see how we care if it happens at the wrong moment that
task->pid might not match pid_nr(pid);

Similarly because the code in posix-cpu-timers goes pid->task->pid it
feels like there should be a problem.  But as the code that works with
PIDTYPE_PID is only available within the thread group, and as de_thread
kills all of the other threads before it makes any changes of this
kind the race can not happen.

In short I don't think this change will introduce any regressions.

Eric W. Biederman (2):
      rculist: Add hlists_swap_heads_rcu
      proc: Ensure we see the exit of each process tid exactly once

 fs/exec.c               |  5 +----
 include/linux/pid.h     |  1 +
 include/linux/rculist.h | 21 +++++++++++++++++++++
 kernel/pid.c            | 19 +++++++++++++++++++
 4 files changed, 42 insertions(+), 4 deletions(-)

Eric