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Date: Thu, 20 Aug 2009 10:39:12 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: Paul Menage <menage@google.com>
CC: akpm@linux-foundation.org, bblum@andrew.cmu.edu,
       linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org
Subject: Re: [PATCH 7/8] Adds functionality to read/write lock CLONE_THREAD
 fork()ing per-threadgroup
References: <20090818235059.22531.42618.stgit@menage.mtv.corp.google.com> <20090818235843.22531.57310.stgit@menage.mtv.corp.google.com>
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Paul Menage wrote:
> From: Ben Blum <bblum@google.com>
> 
> 
> Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup
> 
> This patch adds an rwsem that lives in a threadgroup's sighand_struct (next to
> the sighand's atomic count, to piggyback on its cacheline), and two functions
> in kernel/cgroup.c (for now) for easily+safely obtaining and releasing it. If
> another part of the kernel later wants to use such a locking mechanism, the
> CONFIG_CGROUPS ifdefs should be changed to a higher-up flag that CGROUPS and
> the other system would both depend on, and the lock/unlock functions could be
> moved to sched.c or so.
> 
> Signed-off-by: Ben Blum <bblum@google.com>
> Signed-off-by: Paul Menage <menage@google.com>
>

Looks fine to me.

Acked-by: Li Zefan <lizf@cn.fujitsu.com>

... 

> +struct sighand_struct *threadgroup_fork_lock(struct task_struct *tsk)
> +{
> +	struct sighand_struct *sighand;
> +	struct task_struct *p;
> +
> +	/* tasklist lock protects sighand_struct's disappearance in exit(). */
> +	read_lock(&tasklist_lock);
> +	if (likely(tsk->sighand)) {
> +		/* simple case - check the thread we were given first */
> +		sighand = tsk->sighand;
> +	} else {
> +		sighand = NULL;
> +		/*
> +		 * tsk is exiting; try to find another thread in the group
> +		 * whose sighand pointer is still alive.
> +		 */
> +		rcu_read_lock();

since we are holding tasklist_lock, I think we don't need to
take rcu lock?

> +		list_for_each_entry_rcu(p, &tsk->thread_group, thread_group) {
> +			if (p->sighand) {
> +				sighand = tsk->sighand;

s/tsk->sighand/p->sighand

> +				break;
> +			}
> +		}
> +		rcu_read_unlock();
> +	}