linux-kernel - Re: Documentation of locking needs when working with lists?

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Message-ID: <20250513224229.78c95926@pumpkin>
Date: Tue, 13 May 2025 22:42:29 +0100
From: David Laight <david.laight.linux@...il.com>
To: Heiner Kallweit <hkallweit1@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...nel.org>, Linux Kernel
 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Documentation of locking needs when working with lists?

On Sat, 10 May 2025 10:46:32 +0200
Heiner Kallweit <hkallweit1@...il.com> wrote:

> Even though lists are used everywhere, I was surprised not being able to find
> documentation about which operations need locking, and which ones are safe
> lock-less.
> 
> My case:
> I have a list where the only operation is adding entries.
> It's clear that adding entries has to be serialized.
> Question is whether a list_for_each_entry is safe lock-less.
> 
> Looking at the code I *think* it's safe, under the precondition that
> reading/writing pointers is atomic.
> 
> Any hint or documentation link would be appreciated. Thanks!
> 

Well, somewhere you must have code that destroys the list.
That must be locked against anything that traverses it ...

But if you have a singly linked list and keep a pointer to the last
entry in order to append entries then the list traversal code wont
explode.

However, on many architectures, you need memory sequencing instructions
to guarantee the list traversal code reads the expected contents of
newly added (or any other recently changed) items.
Read up on the semantics acquire and release operations.
Locks will give the required guarantee.

Most lockless lists in the kernel use 'rcu'.

	David