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

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Message-ID: <20090416134040.GB6532@redhat.com>
Date:	Thu, 16 Apr 2009 15:40:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com, jmorris@...ei.org, roland@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rework/fix is_single_threaded()

On 04/15, Andrew Morton wrote:
>
> On Mon, 13 Apr 2009 23:45:13 +0200
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > - Fix the comment, is_single_threaded(p) actually means that nobody shares
> >   ->mm with p.
> >
> >   I think this helper should be renamed, and it should not have arguments.
> >   With or without this patch it must not be used unless p == current,
> >   otherwise we can't safely use p->signal or p->mm.
> >
> > - "if (atomic_read(&p->signal->count) != 1)" is not right when we have a
> >   zombie group leader, use signal->live instead.
> >
> > - Add PF_KTHREAD check to skip kernel threads which may borrow p->mm,
> >   otherwise we can return the wrong "false".
> >
> > - Use for_each_process() instead of do_each_thread(), all threads must use
> >   the same ->mm.
> >
> > - Use down_write(mm->mmap_sem) + rcu_read_lock() instead of tasklist_lock
> >   to iterate over the process list. If there is another CLONE_VM process
> >   it can't pass exit_mm() which takes the same mm->mmap_sem. We can miss
> >   a freshly forked CLONE_VM task, but this doesn't matter because we must
> >   see its parent and return false.
>
> hum.  I'm searching for a reason to merge this into 2.6.30 but I'm not
> seeing one?

No reasons to merge.

Also, David seem to have valid concerns about using ->mmap_sem. Most probably
this patch should be changed anyway. Let's see what David thinks.

Oleg.

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