From: Boqun Feng <boqun.feng@gmail.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
lkmm@lists.linux.dev, Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
Waiman Long <longman@redhat.com>,
Davidlohr Bueso <dave@stgolabs.net>,
"Paul E. McKenney" <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Frederic Weisbecker <frederic@kernel.org>,
Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
Joel Fernandes <joelagnelf@nvidia.com>,
Uladzislau Rezki <urezki@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Zqiang <qiang.zhang@linux.dev>, Breno Leitao <leitao@debian.org>,
aeh@meta.com, netdev@vger.kernel.org, edumazet@google.com,
jhs@mojatatu.com, kernel-team@meta.com,
Erik Lundgren <elundgren@meta.com>
Subject: Re: [PATCH 0/8] Introduce simple hazard pointers for lockdep
Date: Thu, 26 Jun 2025 08:47:25 -0700 [thread overview]
Message-ID: <aF1rjV8XQozi7hXB@Mac.home> (raw)
In-Reply-To: <aF0eEfoWVCyoIAgx@infradead.org>
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
next prev parent reply other threads:[~2025-06-26 15:47 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-25 3:10 [PATCH 0/8] Introduce simple hazard pointers for lockdep Boqun Feng
2025-06-25 3:10 ` [PATCH 1/8] Introduce simple hazard pointers Boqun Feng
2025-06-25 10:00 ` Peter Zijlstra
2025-06-25 14:25 ` Mathieu Desnoyers
2025-06-25 15:05 ` Boqun Feng
2025-06-25 15:52 ` Waiman Long
2025-06-25 16:09 ` Boqun Feng
2025-06-25 17:47 ` Waiman Long
2025-06-25 3:10 ` [PATCH 2/8] shazptr: Add refscale test Boqun Feng
2025-06-25 10:02 ` Peter Zijlstra
2025-06-25 3:10 ` [PATCH 3/8] shazptr: Add refscale test for wildcard Boqun Feng
2025-06-25 10:03 ` Peter Zijlstra
2025-06-25 3:10 ` [PATCH 4/8] shazptr: Avoid synchronize_shaptr() busy waiting Boqun Feng
2025-06-25 11:40 ` Peter Zijlstra
2025-06-25 11:56 ` Peter Zijlstra
2025-06-25 13:56 ` Frederic Weisbecker
2025-06-25 15:24 ` Boqun Feng
2025-06-26 13:45 ` Frederic Weisbecker
2025-06-25 3:10 ` [PATCH 5/8] shazptr: Allow skip self scan in synchronize_shaptr() Boqun Feng
2025-06-25 3:10 ` [PATCH 6/8] rcuscale: Allow rcu_scale_ops::get_gp_seq to be NULL Boqun Feng
2025-06-25 3:11 ` [PATCH 7/8] rcuscale: Add tests for simple hazard pointers Boqun Feng
2025-06-25 3:11 ` [PATCH 8/8] locking/lockdep: Use shazptr to protect the key hashlist Boqun Feng
2025-06-25 11:59 ` Peter Zijlstra
2025-06-25 14:18 ` Boqun Feng
2025-07-10 14:06 ` Breno Leitao
2025-07-11 2:31 ` Boqun Feng
2025-06-25 12:05 ` [PATCH 0/8] Introduce simple hazard pointers for lockdep Christoph Hellwig
2025-06-25 14:08 ` Boqun Feng
2025-06-26 10:16 ` Christoph Hellwig
2025-06-26 13:45 ` Mathieu Desnoyers
2025-06-26 15:47 ` Boqun Feng [this message]
2025-06-27 2:56 ` Paul E. McKenney
2025-06-25 12:25 ` Mathieu Desnoyers
2025-06-25 13:21 ` Boqun Feng
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