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Date:   Thu, 23 Sep 2021 06:28:44 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        Matthew Wilcox <willy@infradead.org>,
        John Garry <john.garry@huawei.com>,
        linux-block@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache
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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)