Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

[Sebastian Andrzej Siewior]

On 2022-08-18 16:37:06 [+0200], Vlastimil Babka wrote:
> > The slub fastpath does not interfere with slow path operations and the
> 
> That's true on !PREEMPT_RT because a slowpath operation under
> local_lock_irqsave() will disable interrupts, so there can't be a
> fastpath operation in an interrupt handler appearing in the middle of a
> slowpath operation.
> 
> On PREEMPT_RT local_lock_irqsave() doesn't actually disable interrupts,
> so that can happen. IIRC we learned that the hard way when Mike
> Galbraith was testing early versions of my PREEMPT_RT changes for SLUB.

I think the point is that local_lock_irqsave() does not disable
preemption. So the lock owner (within the local_lock_irqsave() section)
can be preempted and another task can use the fast path which does not
involve the lock and things go boom from here.

Sebastian