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Date: Mon, 7 Oct 2024 12:42:44 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
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	"Paul E. McKenney" <paulmck@kernel.org>,
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	Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
	rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers
Message-ID: <20241007104244.GC4879@noisy.programming.kicks-ass.net>
References: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com>
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 <20241005160444.GA18071@noisy.programming.kicks-ass.net>
 <20241005160723.GI33184@noisy.programming.kicks-ass.net>
 <6e7812f7-c94a-46ba-ba4d-cf4b7b1f60c9@efficios.com>
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In-Reply-To: <6e7812f7-c94a-46ba-ba4d-cf4b7b1f60c9@efficios.com>

On Sat, Oct 05, 2024 at 02:56:26PM -0400, Mathieu Desnoyers wrote:
> On 2024-10-05 18:07, Peter Zijlstra wrote:
> > On Sat, Oct 05, 2024 at 06:04:44PM +0200, Peter Zijlstra wrote:
> > > On Fri, Oct 04, 2024 at 02:27:33PM -0400, Mathieu Desnoyers wrote:
> > 
> > > > +void hp_scan(struct hp_slot __percpu *percpu_slots, void *addr,
> > > > +	     void (*retire_cb)(int cpu, struct hp_slot *slot, void *addr))
> > > > +{
> > > > +	int cpu;
> > > > +
> > > > +	/*
> > > > +	 * Store A precedes hp_scan(): it unpublishes addr (sets it to
> > > > +	 * NULL or to a different value), and thus hides it from hazard
> > > > +	 * pointer readers.
> > > > +	 */
> > 
> > This should probably assert we're in a preemptible context. Otherwise
> > people will start using this in non-preemptible context and then we get
> > to unfuck things later.
> 
> Something like this ?
> 
> +       /* Should only be called from preemptible context. */
> +       WARN_ON_ONCE(in_atomic());

	lockdep_assert_preemption_enabled();

that also checks local IRQ state IIRC.