On 08/15, Luis R. Rodriguez wrote:
>
> On Wed, Aug 13, 2014 at 07:51:01PM +0200, Oleg Nesterov wrote:
> > On 08/12, Luis R. Rodriguez wrote:
> > >
> > > +/* To be used by modules which can take over 30 seconds at probe */
> >
> > Probably the comment should explain that this hack should only be
> > used if the driver is buggy and is wating for "real fix".
> >
> > > +#define module_long_probe_init(initfn) \
> > > + static struct task_struct *__init_thread; \
> > > + static int _long_probe_##initfn(void *arg) \
> > > + { \
> > > + return initfn(); \
> > > + } \
> > > + static inline __init int __long_probe_##initfn(void) \
> > > + { \
> > > + __init_thread = kthread_run(_long_probe_##initfn,\
> > > + NULL, \
> > > + #initfn); \
> > > + if (IS_ERR(__init_thread)) \
> > > + return PTR_ERR(__init_thread); \
> > > + return 0; \
> > > + } \
> > > + module_init(__long_probe_##initfn);
> > > +/* To be used by modules that require module_long_probe_init() */
> > > +#define module_long_probe_exit(exitfn) \
> > > + static inline void __long_probe_##exitfn(void) \
> > > + { \
> > > + exitfn(); \
> > > + if (__init_thread) \
> > > + kthread_stop(__init_thread); \
> > > + } \
> >
> > exitfn() should be called after kthread_stop(), and only if initfn()
> > returns 0. So it should probably do
> >
> > int err = kthread_stop(__init_thread);
> > if (!err)
> > exitfn();
>
> Thanks! With the check for __init_thread as well as it can be
> ERR_PTR(-ENOMEM), ERR_PTR(-EINTR), or NULL (for whatever other
> reason).
Do you mean __long_probe_##exitfn() should also check ERR_PTR(__init_thread)?
I don't think so. If kthread_run() above fails, module_init() should return
the error (it does), so module_exit() won't be called.
> > But there is an additional complication, you can't use __init_thread
> > without get_task_struct(),
>
> Can you elaborate why ? kthread_stop() uses get_task_struct(),
This is too late. This task_struct can be already freed/reused. See below.
> wake_up_process() and finally put_task_struct(), and we're the
> only user of this thread. Also kthread_run() ensures wake_up_process()
> gets called on startup, so not sure where the race would be provided
> all users here and with the respective helpers on buggy drivers.
>
> > so __long_probe_##initfn() can't use
> > kthread_run(). It needs kthread_create() + get_task_struct() + wakeup.
>
> I fail to see why we'd need to add get_task_struct() on
> module_long_probe_init(), can you clarify?
kthread_stop(kthread_run(callback)) is only safe if callback() can not exit
on its own, without checking kthread_should_stop(). And btw that is why
kthread_stop() does get_task_struct()).
If callback() can exit (if it calls do_exit() or simply returns), then nothing
protects this task_struct, it will be freed.
Oleg.
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