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    SubjectRe: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available
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    On Thursday, March 15, 2012, Stephen Boyd wrote:
    > On 03/14/12 16:13, Rafael J. Wysocki wrote:
    > > On Thursday, March 15, 2012, Rafael J. Wysocki wrote:
    > >> On Wednesday, March 14, 2012, Stephen Boyd wrote:
    > >>> On 03/13/12 13:14, Rafael J. Wysocki wrote:
    > >>>> All of those use cases are in fact of the "wait for user space to be thawed
    > >>>> and then load the firmware" type, which I believe may be handled without
    > >>>> changing that code.
    > >>>>
    > >>>> Why don't you make your kthread freezable, for one example?
    > >>>>
    > >>>> Why don't you use a freezable workqueue instead?
    > >>>>
    > >>> If we put it on the freezable workqueue or make it a freezable thread
    > >>> will it work?
    > >> That depends on what exactly you want to achieve, which isn't entirely clear
    > >> to me at this point.
    > >>
    > >>> In my scenario a wakeup interrupt comes in that wakes us up from
    > >>> suspend. Within that wakeup handler a work item is scheduled to the
    > >>> freezable workqueue. That work item then calls request_firmware().
    > >> That should work.
    > >>
    > >>> It looks like we call schedule() after thawing the workqueues and tasks
    > >>> so the work item could run before usermodehelpers are enabled and then
    > >>> request_firmware() would fail. Do we need something like this (ignore
    > >>> the fact that we call usermodhelper_enable() twice)?
    > >>>
    > >>> diff --git a/kernel/power/process.c b/kernel/power/process.c
    > >>> index 7e42645..61bfa95 100644
    > >>> --- a/kernel/power/process.c
    > >>> +++ b/kernel/power/process.c
    > >>> @@ -187,6 +187,7 @@ void thaw_processes(void)
    > >>> } while_each_thread(g, p);
    > >>> read_unlock(&tasklist_lock);
    > >>>
    > >>> + usermodehelper_enable();
    > >> That would be a reasonable change.
    > >>
    > >>> schedule();
    > >>> printk("done.\n");
    > >>> }
    > >>>
    > >>>
    > >>> Is there a reason we disable usermodehelpers if
    > >>> CONFIG_SUSPEND_FREEZER=n?
    > >> Not really, but CONFIG_SUSPEND_FREEZER=n can only work reliably in a
    > >> very limited set of cases, so I don't think it's even worth making the
    > >> general code depend on it.
    > >>
    > >> I'd actually prefer to remove CONFIG_SUSPEND_FREEZER altogether,
    > >> because it's not very useful nowadays (probably isn't useful at all).
    > >>
    > >>> Should we do this instead so that
    > >>> usermodehelpers are only disabled if we freeze userspace? Also what is
    > >>> that schedule() call in thaw_kernel_threads() for? It looks like we'll
    > >>> call schedule between kernel thread thawing and userspace thawing.
    > >> Which is OK, I think.
    > > Moreover, thaw_kernel_threads() is _only_ called by (a) freeze_kernel_threads()
    > > on error and (b) user-space hibernate interface in kernel/power/user.c
    > > (and please read the comment in there describing what it's there for, which
    > > also explains why the schedule() call in there is necessary).
    >
    > Exactly. So in case (a) when the error occurs we'll have this call flow:
    >
    > usermodehelpers_disable()
    > suspend_freeze_processes()
    > freeze_processes()
    > freeze_kernel_threads()
    > try_to_freeze_tasks() <-- returns error
    > thaw_kernel_threads()
    > schedule()
    > thaw_processes()
    > usermodehelpers_enable()
    >
    > Shouldn't we schedule only after we thaw all processes (not just tasks)?
    > Otherwise we may run a kernel thread before userspace is thawed?

    Yes, we may, but that isn't wrong, is it?

    Only a few kernel threads are freezable, so definitely kernel threads
    can run while user space is frozen.

    Thanks,
    Rafael


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