Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Aug 16, 2019 at 1:49 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Can we finally put a foot down and tell compiler and standard committee
> people to stop this insanity?

It's already effectively done.

Yes, values can be read from memory multiple times if they need
reloading. So "READ_ONCE()" when the value can change is a damn good
idea.

But it should only be used if the value *can* change. Inside a locked
region it is actively pointless and misleading.

Similarly, WRITE_ONCE() should only be used if you have a _reason_ for
using it (notably if you're not holding a lock).

If people use READ_ONCE/WRITE_ONCE when there are locks that prevent
the values from changing, they are only making the code illegible.
Don't do it.

But in the *absence* of locking, READ_ONCE/WRITE_ONCE is usually a
good thing.  The READ_ONCE actually tends to matter, because even if
the value is used only once at a source level, the compiler *could*
decide to do something else.

The WRITE_ONCE() may or may not matter (afaik, thanks to concurrency,
modern C standard does not allow optimistic writes anyway, and we
wouldn't really accept such a compiler option if it did).

But if the write is done without locking, it's good practice just to
show you are aware of the whole "done without locking" part.

              Linus