linux-kernel - Re: [PATCH] rework/fix is_single

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Message-ID: <20090709212555.GA9861@redhat.com>
Date:	Thu, 9 Jul 2009 23:25:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, jmorris@...ei.org,
	roland@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rework/fix is_single_threaded()

On 07/09, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > David, do you still dislike ->mmap_sem? I didn't see other objections,
> > and again, imho tasklist_lock is worse.
>
> My main objection to taking mmap_sem is that it restricts where the function
> can be used.  It can't, for example, be called by anyone holding a spinlock.

Yes, it is might_sleep().

> Furthermore, the more locks, the more chance of someone accidentally
> deadlocking something.

The current code is not lockless too, tasklist_lock is not free and
can lead too deadlocks as well.

Anyway. I agree it is better to avoid ->mmap_sem, I'll send the patch
in a minute. But I'd really like to do this in a separate patch, the
change is subtle and needs a changelog at least.

> But apart from that, go for it.

Great, thanks. Then I'll make the patch on top.

Oleg.

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