linux-kernel - Re: [PATCH] change current_is_single_threaded() to use for_each

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Message-ID: <20151007115404.GA20428@redhat.com>
Date:	Wed, 7 Oct 2015 13:54:04 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] change current_is_single_threaded() to use
	for_each_thread()

On 10/07, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Change current_is_single_threaded() to use for_each_thread() rather
> > than deprecated while_each_thread().
>
> Wouldn't the old way be more efficient, given there's always going to be at
> least one thread?

Yes, unfortunately. Before this change (in the likely case) we never
do next_thread(), after this patch we always do list_first_entry().

> Granted, it's not much of an efficiency boost...

Yes, and while_each_thread() is buggy. See the changelog in 0c740d0afc
"introduce for_each_thread() to replace the buggy while_each_thread().

Yes, the race is almost purely theoretical, but still.

To clarify, we had some "real" bugs fixed by s/while_each_thread/
for_each_thread/, but only because while_each_thread() was used wrongly.
for_each_thread() is more safe, it only needs the stable task_struct.

Oleg.

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