Re: Documentation of locking needs when working with lists?

[Heiner Kallweit]

On 10.05.2025 15:20, Greg Kroah-Hartman wrote:
> On Sat, May 10, 2025 at 10:46:32AM +0200, Heiner Kallweit 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!
> 
> You MUST have locking for your list if you have multiple processes
> accessing it at the same time.
> 
Thanks. Sure, I need locking for the writers (list_add_tail).
Question is about the reader, list_for_each_entry.
Last step in list_add_tail() is WRITE_ONCE(prev->next, new);
list_next_entry() reads the "next" member of the iterator.

Therefore I think list_next_entry() always sees a consistent
state, either the old or the new value of the "next" pointer.
So I don't see a need for locking list_for_each_entry().

If there is such a need, I'd be interested in the potential
race scenario.

> good luck!
> 
> greg k-h