linux-kernel - Re: [PATCH] zap_pid_ns_processes: clear TIF_NOTIFY_SIGNAL along with TIF

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Date: Thu, 13 Jun 2024 17:30:22 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Rachel Menge <rachelmenge@...ux.microsoft.com>,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	Wei Fu <fuweid89@...il.com>, apais@...ux.microsoft.com,
	Sudhanva Huruli <Sudhanva.Huruli@...rosoft.com>,
	Jens Axboe <axboe@...nel.dk>,
	Christian Brauner <brauner@...nel.org>,
	Mike Christie <michael.christie@...cle.com>,
	Joel Granados <j.granados@...sung.com>,
	Mateusz Guzik <mjguzik@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH] zap_pid_ns_processes: clear TIF_NOTIFY_SIGNAL along with
 TIF_SIGPENDING

On 06/13, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > kernel_wait4() doesn't sleep and returns -EINTR if there is no
> > eligible child and signal_pending() is true.
> >
> > That is why zap_pid_ns_processes() clears TIF_SIGPENDING but this is not
> > enough, it should also clear TIF_NOTIFY_SIGNAL to make signal_pending()
> > return false and avoid a busy-wait loop.
>
> I took a look through the code.  It used to be that TIF_NOTIFY_SIGNAL
> was all about waking up a task so that task_work_run can be used.
> io_uring still mostly uses it that way.  There is also a use in
> kthread_stop that just uses it as a TIF_SIGPENDING without having a
> pending signal.
>
> At the point in do_exit where exit_notify and thus zap_pid_ns_processes
> is called I can't possibly see a use for TIF_NOTIFY_SIGNAL.
> exit_task_work, exit_signals, and io_uring_cancel have all been called.
>
> So TIF_NOTIFY_SIGNAL should be spurious at this point and safe to clear.
> Why it remains set is a mystery to me.

because exit_task_work() -> task_work_run() doesn't clear TIF_NOTIFY_SIGNAL.

So yes, it is spurious, but to me a possible TIF_SIGPENDING is even more
"spurious". See my reply to Wei.

We don't need to clear TIF_NOTIFY_SIGNAL inside the loop, task_work_addd()
can't succeed after exit_task_work() sets ->task_works =&work_exited, but
this is another story and this doesn't (well, shouldn't) differ from
TIF_SIGPENDING.

> If I had infinite time and energy the ideal is to rework the pid
> namespace exit logic

Perhaps  in this case you could take a look at the next loop waiting for
pid_ns->pid_allocated == init_pids ;)

I always hated the fact that the the exiting sub-namespace init can
"hang forever" if this namespace has the tasks injected from the parent
namespace. And I had enough hard-to-debug internal bug reports which
blamed the kernel.

> This active waiting is weird and non-standard in the kernel and winds up
> causeing a problem every couple of years because of that.

Agreed.

Oleg.