Re: [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment

[Mathieu Desnoyers]

On 2024-03-01 10:49, Steven Rostedt wrote:
> On Fri, 1 Mar 2024 13:37:18 +0800
> linke <lilinke99@xxxxxx> wrote:
>
> > > So basically you are worried about read-tearing?
> > >
> > > That wasn't mentioned in the change log.
> >
> > Yes. Sorry for making this confused, I am not very familiar with this and
> > still learning.
>
> No problem. We all have to learn this anyway.
>
> > > Funny part is, if the above timestamp read did a tear, then this would
> > > definitely not match, and would return the correct value. That is, the
> > > buffer is not empty because the only way for this to get corrupted is if
> > > something is in the process of writing to it.
> >
> > I agree with you here.
> >
> > 	commit = rb_page_commit(commit_page);
> >
> > But if commit_page above is the result of a torn read, the commit field
> > read by rb_page_commit() may not represent a valid value.
>
> But commit_page is a word length, and I will argue that any compiler that
> tears "long" words is broken. ;-)

[ For those tuning in, we are discussing ring_buffer_iter_empty()
  "commit_page = cpu_buffer->commit_page;" racy load. ]

I counter-argue that real-world compilers *are* broken based on your
personal definition, but we have to deal with them, as documented
in Documentation/memory-barriers.txt (see below).

What is the added overhead of using a READ_ONCE() there ? Why are
we wasting effort trying to guess the compiler behavior if the
real-world performance impact is insignificant ?

Quote from memory-barrier.txt explaining the purpose of {READ,WRITE}_ONCE():

"(*) For aligned memory locations whose size allows them to be accessed
     with a single memory-reference instruction, prevents "load tearing"
     and "store tearing," in which a single large access is replaced by
     multiple smaller accesses."

I agree that {READ,WRITE}_ONCE() are really not needed at initialization,
when there are demonstrably no concurrent accesses to the data

But trying to eliminate {READ,WRITE}_ONCE() on concurrently accessed fields
just adds complexity, prevents static analyzers to properly understand the
code and report issues, and just obfuscates the code.

Thanks,

Mathieu

> > In this case, READ_ONCE() is only needed for the commit_page.
>
> But we can at least keep the READ_ONCE() on the commit_page just because it
> is used in the next instruction.
>
> -- Steve

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