Date: Fri, 10 Apr 2015 19:50:48 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <ktkhai@parallels.com>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.cz>, Rik van Riel <riel@redhat.com>,
        Ionut Alexa <ionut.m.alexa@gmail.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [PATCH] exit: Use read lock for do_notify_parent() instead of
	write lock
Message-ID: <20150410175048.GA23971@redhat.com>
References: <1428602348.12166.29.camel@parallels.com>
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Kirill,

I'll try to read this patch tomorrow, currently I am hopelessly buried
in user-space programming :/

But I have to admit that so far I dislike this patch very much. It adds
a lot of complications and for what?

Yes, yes, yes. tasklist_lock is another BKL and must die. We need the
per-process lock. Until then I do not think the hacks like this make
any sense, unless you have the "real" workload with before/after
performance numbers.

On 04/09, Kirill Tkhai wrote:
>
> I suggest to execute do_notify_parent() under read_lock(). It allows more tasks
> to use it in parallel. Read lock gives enough guarantees for us: child's parent
> won't change during the notification.

Well, write_unlock() + read_lock() is not nice too...

> include/asm-generic/qrwlock.h:
> static inline void queue_reduce_locked_write_to_read(struct qrwlock *lock)
> {
> 	smp_mb__before_atomic();
> 	atomic_add(_QR_BIAS - _QW_LOCKED, &lock->cnts);
> }

Yes, downgrade() will be better.

Still, this only removes do_notify_parent() from the write_lock'ed section.

(lets ignore kill_orphaned_pgrp(), we want to make will_become_orphaned_pgrp
 lockless. Look at get_signal).

And this changes the rules: currently ->exit_state is stable under read_lock,
except -> EXIT_DEAD transition. OK, this is probably fine, but we need to
recheck. At least this was certainly wrong some time before iirc.

> @@ -594,7 +597,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>
>  	write_lock_irq(&tasklist_lock);
>  	forget_original_parent(tsk, &dead);
> +	tsk->exit_state = EXIT_NOTIFY;
> +	write_unlock_irq(&tasklist_lock);

And unless I missed something this EXIT_NOTIFY turns the concurrent
do_wait() into the busy-wait loop.

Now suppose that CONFIG_SMP=n and the rt parent preempts the exiting
child right after it drops tasklist: deadlock?

> +	read_lock(&tasklist_lock);
>  	if (group_dead)
>  		kill_orphaned_pgrp(tsk->group_leader, NULL);
>
> @@ -612,13 +618,14 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  	}
>
>  	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;

This needs WRITE_ONCE(). Otherwise gcc can do, say,

	tsk->exit_state = EXIT_ZOMBIE;
	if (autoreap)
		tsk->exit_state = EXIT_DEAD;

which will lead to kernel crash (both parent and child can release this
task).


> -	if (tsk->exit_state == EXIT_DEAD)
> +	smp_wmb(); /* Pairs with read_lock() in do_wait() */

Why? this barries looks unnecessary.

OTOH. We need to set EXIT_XXX before __wake_up_parent(). OK, OK, we do not
because of the busy-wait loop, but busy-wait is not an option.

> @@ -1317,6 +1324,13 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
>  		return 0;
>  	}
>
> +	if (unlikely(exit_state == EXIT_NOTIFY)) {
> +		if (wo->wo_flags & WNOHANG)
> +			return 0;
> +		read_unlock(&tasklist_lock);
> +		return -REPEAT_DOWAIT;
> +	}

No, no, no. If you do something like this, please (ab)use wo->notask_error.
And wait_consider_task() should continue after that,

Oleg.

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