linux-kernel - [PATCH -mm 0/3] proc: task->signal can't be NULL

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Message-ID: <20100322184127.GA3952@redhat.com>
Date:	Mon, 22 Mar 2010 19:41:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH -mm 0/3] proc: task->signal can't be NULL

With the recent changes in -mm it is always safe to dereference
task->signal. It can't be NULL and it is pinned to task_struct.

fs/proc becomes the only valid user of signal->count which should
either die or become "int nr_threads".

Alexey, Eric.

Can't we kill this counter? Afaics, get_nr_threads() doesn't need to
be "precise", we probably can estimate the number of threads using
signal->live (yes sure, we can't use ->live as nr_threads).

Except: first_tid() uses get_nr_threads() for optimization. Is this
optimization really important? Afaics, it only helps in the unlikely
case, probably in that case the extra lockless while_each_thread()
doesn't hurt.

IOW, how about

	--- a/fs/proc/base.c
	+++ b/fs/proc/base.c
	@@ -3071,11 +3071,6 @@ static struct task_struct *first_tid(str
				goto found;
		}

	-	/* If nr exceeds the number of threads there is nothing todo */
	-	pos = NULL;
	-	if (nr && nr >= get_nr_threads(leader))
	-		goto out;
	-
		/* If we haven't found our starting place yet start
		 * with the leader and walk nr threads forward.
		 */

?

Not that I think it is terribly important to kill this counter, and
probably signal->nr_threads can make sense anyway, so far I am just
curious.

Oleg.

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