[RFC please help] membarrier: Rewrite sync_core_before_usermode()

[Russell King - ARM Linux admin]

On Wed, Dec 30, 2020 at 12:33:02PM +1000, Nicholas Piggin wrote:
> Excerpts from Russell King - ARM Linux admin's message of December 29, 2020 8:44 pm:
> > On Tue, Dec 29, 2020 at 01:09:12PM +1000, Nicholas Piggin wrote:
> >> I think it should certainly be documented in terms of what guarantees
> >> it provides to application, _not_ the kinds of instructions it may or
> >> may not induce the core to execute. And if existing API can't be
> >> re-documented sanely, then deprecatd and new ones added that DTRT.
> >> Possibly under a new system call, if arch's like ARM want a range
> >> flush and we don't want to expand the multiplexing behaviour of
> >> membarrier even more (sigh).
> > 
> > The 32-bit ARM sys_cacheflush() is there only to support self-modifying
> > code, and takes whatever actions are necessary to support that.
> > Exactly what actions it takes are cache implementation specific, and
> > should be of no concern to the caller, but the underlying thing is...
> > it's to support self-modifying code.
> 
>    Caveat
>        cacheflush()  should  not  be used in programs intended to be portable.
>        On Linux, this call first appeared on the MIPS architecture, but  nowa‐
>        days, Linux provides a cacheflush() system call on some other architec‐
>        tures, but with different arguments.
> 
> What a disaster. Another badly designed interface, although it didn't 
> originate in Linux it sounds like we weren't to be outdone so
> we messed it up even worse.
> 
> flushing caches is neither necessary nor sufficient for code modification
> on many processors. Maybe some old MIPS specific private thing was fine,
> but certainly before it grew to other architectures, somebody should 
> have thought for more than 2 minutes about it. Sigh.

WARNING: You are bordering on being objectionable and offensive with
that comment.

The ARM interface was designed by me back in the very early days of
Linux, probably while you were still in dypers, based on what was
known at the time.  Back in the early 2000s, ideas such as relaxed
memory ordering were not known.  All there was was one level of
harvard cache.

So, juts shut up with your insults.

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