Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache

[Jakub Kicinski]

On Wed, 22 Sep 2021 06:58:00 -0600 Jens Axboe wrote:
> > I considered only case 2) when writing code. Well, To support 1),
> > I think there are two ways:
> > 
> >  a) internally call kmem_cache_free when in_interrupt() is true
> >  b) caller must disable interrupt when freeing
> > 
> > I think a) is okay, how do you think?  
> 
> If the API doesn't support freeing from interrupts, then I'd make that
> the rule. Caller should know better if that can happen, and then just
> use kmem_cache_free() if in a problematic context. That avoids polluting
> the fast path with that check. I'd still make it a WARN_ON_ONCE() as
> described and it can get removed later, hopefully.

Shooting from the hip a little but if I'm getting the context right
this is all very similar to the skb cache so lockdep_assert_in_softirq()
may be useful:

/*
 * Acceptable for protecting per-CPU resources accessed from BH.
 * Much like in_softirq() - semantics are ambiguous, use carefully.
 */
#define lockdep_assert_in_softirq()					\
do {									\
	WARN_ON_ONCE(__lockdep_enabled			&&		\
		     (!in_softirq() || in_irq() || in_nmi()));		\
} while (0)