linux-kernel - Re: [PATCH v2 1/4] introduce for_each_thread() to replace the buggy while_each

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Message-ID: <20131204134917.GA7251@redhat.com>
Date:	Wed, 4 Dec 2013 14:49:17 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Mandeep Singh Baines <msb@...omium.org>,
	"Ma, Xindong" <xindong.ma@...el.com>,
	Michal Hocko <mhocko@...e.cz>,
	Sameer Nanda <snanda@...omium.org>,
	Sergey Dyasly <dserrg@...il.com>,
	"Tu, Xiaobing" <xiaobing.tu@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] introduce for_each_thread() to replace the
	buggy while_each_thread()

On 12/04, Frederic Weisbecker wrote:
>
> On Wed, Dec 04, 2013 at 02:04:09PM +0100, Oleg Nesterov wrote:
>
> > For example, do/while_each_thread() always
> > sees at least one task, while for_each_thread() can do nothing if
> > the whole thread group has died.
>
> Would it be safe to have for_each_thread_continue() instead?

Yes, and no.

Yes, perhaps we will need for_each_thread_continue(). I am not sure
yet. And note that, say, check_hung_uninterruptible_tasks() already
does _continue if fact, although it is still not clear to me if we
actually need this helper.

But no, _continue() can't help if the whole thread group has died,
we simply can not continue.

Note also that _continue() can't be safely used lockless, unless
you verify pid_alive() or something similar.

And,

> Yeah if the conversion needs careful audit, it makes sense to switch incrementally.

Yes. For example the case above. If someone does

	do
		do_something(t);
	while_each_thread(g, t);

we should check that it can tolerate the case when do_something()
won't be called at all, or ensure that this is not possible.

Oleg.

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