linux-kernel - Re: [PATCH 1/3] make task_struct->signal immutable/refcountable

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Message-Id: <20100409193816.A3C36B082@magilla.sf.frob.com>
Date:	Fri,  9 Apr 2010 12:38:16 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] make task_struct->signal immutable/refcountable

Acked-by: Roland McGrath <roland@...hat.com>
but some comments:

> This patch adds the reference counter, sigcnt, into signal_struct. This
> reference is owned by task_struct and it is dropped in __put_task_struct().

I'm not thrilled about the field name, but whatever.  We now have three
atomic counts with subtly different meanings, and no comments anywhere
really explaining it all.

* live: created and not yet begun to exit
* count: not yet reaped
* sigcnt: task_struct not yet freed

> Perhaps it makes sense to export get/put_signal_struct() later, but
> currently I don't see the immediate reason.

We should never export those at all.  In fact, I would not even make it a
separate inline.  We're here to simplify the lifetime rules, not give new
ways to complicate them.  If anybody wants to keep a signal_struct alive,
they can do it with get_task_struct().

> 	- with or without this patch signal_struct->count should go away,
> 	  or at least it should be "int nr_threads" for fs/proc. This will
> 	  be addressed later.

Indeed.  IMHO adding yet another atomic count here is OK only on the
proviso that we will be getting rid of the old one RSN.

Thanks,
Roland
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