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    SubjectRe: [PATCH v3 1/3] init / kthread: add module_long_probe_init() and module_long_probe_exit()
    At Sat, 16 Aug 2014 04:50:07 +0200,
    Luis R. Rodriguez wrote:
    >
    > On Fri, Aug 15, 2014 at 04:39:02PM +0200, Oleg Nesterov wrote:
    > > 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.
    >
    > Good point.
    >
    > > > > 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.
    >
    > OK thanks, yeah I see the issue now, and I was able to create a null
    > pointer dereference by simply calling schedule() quite a bit, will
    > roll in the required fixes, but come to think of it if there are
    > other uses (I haven't SmPLd grep'd for grammar uses yet) perhaps
    > generic helpers would be good? kthread_run_alloc() kthread_run_free().

    How about just increasing/decreasing the module count for blocking the
    exit call? For example:

    #define module_long_probe_init(initfn) \
    static int _long_probe_##initfn(void *arg) \
    { \
    int ret = initfn(); \
    module_put(THIS_MODULE); \
    return ret; \
    } \
    static inline __init int __long_probe_##initfn(void) \
    { \
    struct task_struct *__init_thread; \
    __module_get(THIS_MODULE); \
    __init_thread = kthread_run(_long_probe_##initfn,\
    NULL, \
    #initfn); \
    if (IS_ERR(__init_thread)) { \
    module_put(THIS_MODULE); \
    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) \
    module_exit(exitfn);



    Takashi


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