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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-09-27 19:23, Linus Torvalds wrote:
> On Fri, 27 Sept 2024 at 10:17, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> The barrier() is ineffective at fixing the issue.
>> It does not prevent the compiler CSE from losing the
>> address dependency:
> 
> Ok. Thanks for actually specifying code.
> 
> That needs to be
> 
>   (a) in a comment

OK. I'll add the code/asm examples to the comment above ADDRESS_EQ().

> 
>   (b) the value barrier needs to be on *both* values so that the order
> of the equality testing doesn't matter.

If we use OPTIMIZER_HIDE_VAR() on both parameters, it indeed minimizes
the odds that someone get the order wrong, but it disallows using
ADDRESS_EQ() with a constant parameter
(e.g. ADDRESS_EQ(ptr, &mystruct)), which would be nice to cover. It
works fine with using OPTIMIZER_HIDE_VAR() on the first argument,
but it opens the door to misuses.

Perhaps there is a trick with compiler builtins we could do to only
use OPTIMIZER_HIDE_VAR() on non-constant arguments, but I can't get
it to work so far.

> 
>> I'm preparing a small series that aims to show how a minimal
>> hazard pointer implementation can help improve common scenarios:
> 
> I want actual numbers on real loads. Just so you know.  Not "this can
> help". But "this actually really _does_ help".

Noted, thanks!

Mathieu

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