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Date: Fri, 17 Jul 2009 18:43:46 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Catalin Marinas <catalin.marinas@arm.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] kmemleak: Scan all thread stacks
Message-ID: <20090717164346.GC5131@elte.hu>
References: <20090717093359.8288.45464.stgit@pc1117.cambridge.arm.com>
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* Catalin Marinas <catalin.marinas@arm.com> wrote:

> This patch changes the for_each_process() loop with the
> do_each_thread()/while_each_thread() pair. It also replaces the
> read_lock(&tasklist_lock) with rcu_read_lock() and task_lock(p).
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
> 
> My questions:
> 
> 1. Is it correct that for_each_process() used currently by kmemleak may
>    not loop through all the possible kernel thread stacks?

yes. You need the full, all tasks variant:

        do_each_thread(g, p) {
        ...
        } while_each_thread(g, p);


> 2. Is it safe to use rcu_read_lock() and task_lock() when scanning the
>    corresponding kernel stack (thread_info structure)? The loop doesn't
>    do any modification to the task list. The reason for this is to
>    allow kernel preemption when scanning the stacks.

you cannot generally preempt while holding the RCU read-lock.

	Ingo