linux-kernel - Re: [RFC PATCH 03/30] Lazy percpu counters

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Date:   Thu, 1 Sep 2022 20:59:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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Subject: Re: [RFC PATCH 03/30] Lazy percpu counters

On Thu, Sep 01, 2022 at 10:32:19AM -0400, Kent Overstreet wrote:
> On Thu, Sep 01, 2022 at 08:51:31AM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:
> > > +static void lazy_percpu_counter_switch_to_pcpu(struct raw_lazy_percpu_counter *c)
> > > +{
> > > +	u64 __percpu *pcpu_v = alloc_percpu_gfp(u64, GFP_ATOMIC|__GFP_NOWARN);
> > 
> > Realize that this is incorrect when used under a raw_spinlock_t.
> 
> Can you elaborate?

required lock order: raw_spinlock_t < spinlock_t < mutex

allocators lives at spinlock_t.

Also see CONFIG_PROVE_RAW_LOCK_NESTING and there might be a document
mentioning all this somewhere.

Additionally, this (obviously) also isn't NMI safe.