linux-kernel - Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

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Message-ID: <6e7812f7-c94a-46ba-ba4d-cf4b7b1f60c9@efficios.com>
Date: Sat, 5 Oct 2024 14:56:26 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Nicholas Piggin <npiggin@...il.com>, Michael Ellerman <mpe@...erman.id.au>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Paul E. McKenney" <paulmck@...nel.org>, Will Deacon <will@...nel.org>,
 Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>,
 Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Frederic Weisbecker <frederic@...nel.org>,
 Joel Fernandes <joel@...lfernandes.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan
 <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
 Mateusz Guzik <mjguzik@...il.com>,
 Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, rcu@...r.kernel.org,
 linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

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());

> 
>>> +
>>> +	if (!addr)
>>> +		return;
>>> +	/* Memory ordering: Store A before Load B. */
>>> +	smp_mb();
>>> +	/* Scan all CPUs slots. */
>>> +	for_each_possible_cpu(cpu) {
>>> +		struct hp_slot *slot = per_cpu_ptr(percpu_slots, cpu);
>>> +
>>> +		if (retire_cb && smp_load_acquire(&slot->addr) == addr)	/* Load B */
>>> +			retire_cb(cpu, slot, addr);
>>
>> Is retirce_cb allowed to cmpxchg the thing?

Renaming retire_cb to "on_match_cb". Whatever the callback does needs to
be done with knowledge of the slot user (e.g. IPI).

>>
>>> +		/* Busy-wait if node is found. */
>>> +		while ((smp_load_acquire(&slot->addr)) == addr)	/* Load B */
>>> +			cpu_relax();
>>
>> This really should be using smp_cond_load_acquire()

Done,

Thanks,

Mathieu

>>
>>> +	}
>>> +}

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com