linux-kernel - linux/kernel/sched.c: context

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Message-Id: <201009081433.35019.fabio@metanix.org>
Date:	Wed, 8 Sep 2010 14:33:34 +0200
From:	fabio de francesco <fabio@...anix.org>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: linux/kernel/sched.c: context_switch().

There must be something I am missing in sched.c at context_switch() function 
definition. 

There I can read the following code:

static inline void
context_switch(struct rq *rq, struct task_struct *prev,
               struct task_struct *next)
{
	...
	struct mm_struct *mm;
	mm = next->mm;
	if (likely(!mm)) {
                next->active_mm = oldmm;
                atomic_inc(&oldmm->mm_count);
                enter_lazy_tlb(oldmm, next);
        } else
                switch_mm(oldmm, mm, next);
	...
}

May someone please explain why the mm pointer is likely supposed to be NULL?

Thanks in advance.

fabio
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