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SubjectRe: [PATCH v3] irq/core: synchronize irq_thread startup
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Hillf,

On Tue, May 03 2022 at 08:42, Hillf Danton wrote:
> On Mon, 02 May 2022 21:24:45 +0200 Thomas Gleixner wrote:
>> On Tue, May 03 2022 at 00:01, Hillf Danton wrote:
>> > + if (!waked)
>> > + wake_threads_waitq(desc);
>>
>> That's a guarantee to make desc->threads_active go negative in the case
>> that the thread was never woken by a hard interrupt handler. IOW, you
>> created a new problem which did not exist before.
>
> The count of active threads would not drop below zero with the change,
> given the comment in __irq_wake_thread(). It is incremented before
> wakeup.

There is no guarantee that the wake-up happens via __irq_wake_thread().
kthread_stop() does a wake-up too, but that obviously _cannot_ increment
the active counter because it does not know about it at all.

create_thread()
thread_fn()
woken = false;
wait_for_wakeup_or_stop() <- Stop is set, no interrupt happened
<- ergo woken == false
if (!woken)
wake_threads_waitq(desc)
atomic_dec_and_test(..) <- underflow

>> The problem discussed here is not a problem in irq_thread(), it's a
>> problem of not reaching this function in the first place. See the on
>> point analysis in Thomas Pfaffs patch.
>
> Well why is the count above zero without wakeup? IOW why is there imbalance
> in count if the irq thread never gets a CPU to run?

Look at kthread() and the condition under which threadfn() is invoked.

Thanks,

tglx

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