linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-ID: <fb85f280-a5b1-487e-a8fa-5e97441991fb@efficios.com>
Date: Wed, 18 Sep 2024 10:27:01 +0200
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
 rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...r.kernel.org
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...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>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Kent Overstreet <kent.overstreet@...il.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
 Neeraj Upadhyay <neeraj.upadhyay@....com>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard
 pointers

On 2024-09-17 16:33, Boqun Feng wrote:
[...]
> 
> The synchronization between readers and updaters is built around "hazard
> pointer slots": a slot readers can use to store a pointer value.
> 
> Reader side protection:
> 
> 1.	Read the value of a pointer to the target data element.
> 2.	Store it to a hazard pointer slot.
> 3.	Enforce full ordering (e.g. smp_mb()).
> 4.	Re-read the original pointer, reset the slot and retry if the
> 	value changed.
> 5.	Otherwise, the continued existence of the target data element
> 	is guaranteed.
> 
> Updater side check:
> 
> 1.	Unpublish the target data element (e.g. setting the pointer
> 	value to NULL).
> 2.	Enforce full ordering.
> 3.	Read the value from a hazard pointer slot.
> 4.	If the value doesn't match the target data element, then this
> 	slot doesn't represent a reference to it.
> 5.	Otherwise, updater needs to re-check (step 3).

Cool! I look forward to see where this is meant to be used. I would
expect it to be a useful tool to speed up reference counting of
things like the mm_struct and for TLB flush IPIs.

On a related note, with a userspace port in mind, the membarrier(2)
syscall can be useful to turn the smp_mb() in (3) from the reader
side into a simple compiler barrier, assuming (2) from the updater
is using membarrier. If IPIs are acceptable (or already required) for
some kernel use-cases, this means a similar asymmetric fence scheme
could be used to speed up readers.

Thanks,

Mathieu

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