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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <bede6c18-45f9-419b-be18-aa167e42ca37@efficios.com>
Date: Sat, 5 Oct 2024 07:42:01 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Frederic Weisbecker <frederic@...nel.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>,
 Peter Zijlstra <peterz@...radead.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>,
 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 13:19, Frederic Weisbecker wrote:
> Le Fri, Oct 04, 2024 at 02:27:33PM -0400, Mathieu Desnoyers a écrit :
>> +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.
>> +	 */
>> +
>> +	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);
>> +		/* Busy-wait if node is found. */
>> +		while ((smp_load_acquire(&slot->addr)) == addr)	/* Load B */
>> +			cpu_relax();
> 
> You agree that having a single possible per-cpu pointer per context and a busy
> waiting update side pointer release can't be a general purpose hazard pointer
> implementation, right? :-)

Of course. This is a minimalist implementation, which can be extended in
various ways, some of which I've implemented as POC in userspace already:

- Increase the number of per-cpu slots available,
- Distinguish between current scan depth target and available
   per-cpu slots,
- Fall-back to reference counter when slots are full,
- Allow scanning for a range of addresses (useful for type-safe
   memory),
- Allow scanning for a set of hazard pointers (scan batching)
   using Bloom filters to probabilistically speed up the comparison
   (not implemented yet).
- Implement a queued blocking wait/wakeup when HP scan must wait
   (not implemented yet).
- Implement a HP-to-refcount promotion triggered by the HP scan
   callback to promote hazard pointers which would be blocked on
   to a reference count increment. (not implemented yet)
- Use hazard pointers + refcount to implement smart pointers, which
   could be useful for Rust. (not implemented yet)

But my general approach is to wait until the use-cases justify adding
features.

Although if you are curious about any of the points listed above,
just ask and I'll be happy to discuss them in more depth.

Thanks,

Mathieu

> 
> Thanks.
> 
>> +	}
>> +}
>> -- 
>> 2.39.2
>>

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