On Wed, Mar 17 2021 at 15:48, Sebastian Andrzej Siewior wrote:

On 2021-03-17 15:38:52 [+0100], Thomas Gleixner wrote:

quoted

thread(irq_A)
  irq_handler(A)
    spin_lock(&foo->lock);

interrupt(irq_B)
  irq_handler(B)
    spin_lock(&foo->lock);

It will not because both threads will wake_up(thread). It is an issue if
- if &foo->lock is shared between a hrtimer and threaded-IRQ
- if &foo->lock is shared between a non-threaded and thread-IRQ
- if &foo->lock is shared between a printk() in hardirq context and
  thread-IRQ as I learned today.

That's the point and it's entirely clear from the above: A is thread
context and B is hard interrupt context and if the lock is shared then
it's busted. Otherwise we would not have this discussion at all.