[Wylug-discuss] The talk on processes from last night....

[Roger Leigh]

Rob Speed <wylug-discuss at robspeed.net> writes:

> uOn Wed, 15 Mar 2006, Roger Leigh wrote:
>>
>> See include/linux/init_task.h (INIT_TASK) for more details.
>> and arch/powerpc/kernel/init_task.c, for example usage.
>
> Naaap - can't find powerpc on my versions of t'kern  (2.4.25,  2.6.9,
> 2.6.15).

It's in 2.6.16-rc6, which has ppc and powerpc arches.  I'm not sure of
the difference.

> If you go by the i386 version, then init_task.c :=

> /*
>  * Initial task structure.
>  *
>  * All other task structs will be allocated on slabs in fork.c
>  */
> struct task_struct init_task = INIT_TASK(init_task);
>
> EXPORT_SYMBOL(init_task);

This is the key part.  init_task is the head of the task table.  Also
see init/main.c.

> doesn't appear to reference pid 0 being the kernel, similarly init_task.h
> doesn't either.

That's correct:

> /*
>  *  INIT_TASK is used to set up the first task table, touch at
>  * your own risk!. Base=0, limit=0x1fffff (=2MB)
>  */
> #define INIT_TASK(tsk)	\
> {									\
> 	.state		= 0,						\
> 	.thread_info	= &init_thread_info,				\
> 	.usage		= ATOMIC_INIT(2),				\
> 	.flags		= 0,						\
> 	.lock_depth	= -1,						\
> 	.prio		= MAX_PRIO-20,					\
> 	.static_prio	= MAX_PRIO-20,					\
> 	.policy		= SCHED_NORMAL,					\
> 	.cpus_allowed	= CPU_MASK_ALL,					\
> 	.mm		= NULL,						\
> 	.active_mm	= &init_mm,					\
> 	.run_list	= LIST_HEAD_INIT(tsk.run_list),			\
> 	.time_slice	= HZ,						\
> 	.tasks		= LIST_HEAD_INIT(tsk.tasks),			\
> 	.ptrace_children= LIST_HEAD_INIT(tsk.ptrace_children),		\
> 	.ptrace_list	= LIST_HEAD_INIT(tsk.ptrace_list),		\
> 	.real_parent	= &tsk,						\
> 	.parent		= &tsk,						\
> 	.children	= LIST_HEAD_INIT(tsk.children),			\
> 	.sibling	= LIST_HEAD_INIT(tsk.sibling),			\
> 	.group_leader	= &tsk,						\
> 	.wait_chldexit	= __WAIT_QUEUE_HEAD_INITIALIZER(tsk.wait_chldexit),\
> 	.real_timer	= {						\
> 		.function	= it_real_fn				\
> 	},								\
> 	.group_info	= &init_groups,					\
> 	.cap_effective	= CAP_INIT_EFF_SET,				\
> 	.cap_inheritable = CAP_INIT_INH_SET,				\
> 	.cap_permitted	= CAP_FULL_SET,					\
> 	.keep_capabilities = 0,						\
> 	.rlim		= INIT_RLIMITS,					\
> 	.user		= INIT_USER,					\
> 	.comm		= "swapper",					\
> 	.thread		= INIT_THREAD,					\
> 	.fs		= &init_fs,					\
> 	.files		= &init_files,					\
> 	.signal		= &init_signals,				\
> 	.sighand	= &init_sighand,				\
> 	.pending	= {						\
> 		.list = LIST_HEAD_INIT(tsk.pending.list),		\
> 		.signal = {{0}}},					\
> 	.blocked	= {{0}},					\
> 	.alloc_lock	= SPIN_LOCK_UNLOCKED,				\
> 	.proc_lock	= SPIN_LOCK_UNLOCKED,				\
> 	.switch_lock	= SPIN_LOCK_UNLOCKED,				\
> 	.journal_info	= NULL,						\
> }

pid is not mentioned in the initialiser list.  It gets default
initialised to 0 (I think; see C99 §6.7.8 clause 10).  See
  include/linux/sched.h (struct task_struct; pid_t pid).
I don't know why it isn't initialised explicitly.  I'm not a kernel
hacker, so I may well be talking rubbish!


Regards,
Roger

-- 
Roger Leigh
                Printing on GNU/Linux?  http://gutenprint.sourceforge.net/
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