linux-kernel - Re: [PATCH 0/8] Introduce simple hazard pointers for lockdep

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Message-ID: <aF1rjV8XQozi7hXB@Mac.home>
Date: Thu, 26 Jun 2025 08:47:25 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, lkmm@...ts.linux.dev,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, Breno Leitao <leitao@...ian.org>,
	aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com,
	jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>
Subject: Re: [PATCH 0/8] Introduce simple hazard pointers for lockdep

On Thu, Jun 26, 2025 at 03:16:49AM -0700, Christoph Hellwig wrote:
> On Wed, Jun 25, 2025 at 07:08:57AM -0700, Boqun Feng wrote:
> > Sure, I will put one for the future version, here is the gist:
> 
> Thanks a lot!
> 
> > The updater's wait can finish immediately if no one is accessing 'a', in
> > other words it doesn't need to wait for reader 2.
> 
> So basically it is the RCU concept, but limited to protecting exactly
> one pointer update per critical section with no ability for the read
> to e.g. acquire a refcount on the objected pointed to by that pointer?

For the current simple hazard pointer, yes. But simple hazard pointers
is easily to extend so support reading:

	{ gp is a global pointer }

	Reader				Updater
	======				=======
	g = shazptr_acquire(p):
	      WRITE_ONCE(*this_cpu_ptr(slot), gp);
	      smp_mb();
	
	if (READ_ONCE(gp) == *this_cpu_ptr(slot)) {
	    // still being protected.
	    <can read gp here>
					to_free = READ_ONCE(gp);
					WRITE_ONCE(gp, new);
					synchronize_shazptr(to_free):
					  smp_mb();
					  // wait on the slot of reader
					  // CPU being 0.
					  READ_ONCE(per_cpu(reader, slot));
	}

	shazptr_clear(g):
	  WRITE_ONCE(*this_cpu_ptr(slot), NULL); // unblock synchronize_shazptr()


Usually the shazptr_acqurie() + "pointer comparison"* is called
shazptr_try_protect().

I will add a document about this in the next version along with other
bits of hazard pointers.

[*]: The pointer comparison is more complicated topic, but Mathieu has
     figured out how to do it correctly:

     https://lore.kernel.org/lkml/20241008135034.1982519-2-mathieu.desnoyers@efficios.com/

Regards,
Boqun