Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT

[Vlastimil Babka]

On 6/14/21 1:33 PM, Vlastimil Babka wrote:
> On 6/14/21 1:16 PM, Sebastian Andrzej Siewior wrote:
> But now that I dig into this in detail, I can see there might be another
> instance of this imbalance bug, if CONFIG_PREEMPTION is disabled, but
> CONFIG_PREEMPT_COUNT is enabled, which seems to be possible in some debug
> scenarios. Because then preempt_disable()/preempt_enable() still manipulate the
> preempt counter and compiling them out in __slab_alloc() will cause imbalance.
> 
> So I think the guards in __slab_alloc() should be using CONFIG_PREEMPT_COUNT
> instead of CONFIG_PREEMPT to be correct on all configs. I dare not remove them
> completely :)

Yep, it's possible to get such scenario with PREEMPT_VOLUNTARY plus
PROVE_LOCKING - CONFIG_PREEMPTION is disabled but CONFIG_PREEMPT_COUNT is
enabled, and RCU then complains in the page allocator due to the unpaired
preempt_disable() before entering it.

I've pushed a new branch revision with this fixed:

https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-local-lock-v2r3