lkml.org 
[lkml]   [2017]   [Oct]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Linux-kernel examples for LKMM recipes
On Tue, 17 Oct 2017, Paul E. McKenney wrote:

> > > > > b. Compilers are permitted to use the "as-if" rule.
> > > > > That is, a compiler can emit whatever code it likes,
> > > > > as long as the results appear just as if the compiler
> > > > > had followed all the relevant rules. To see this,
> > > > > compiler with a high level of optimization and run
> > > > > the debugger on the resulting binary.
> > > >
> > > > You might omit the last sentence. Furthermore, if the accesses don't
> > > > use READ_ONCE/WRITE_ONCE then the code might not get the same result as
> > > > if it had executed in order (even for a single variable!), and if you
> > > > do use READ_ONCE/WRITE_ONCE then the compiler can't emit whatever code
> > > > it likes.
> > >
> > > Ah, I omitted an important qualifier:
> > >
> > > b. Compilers are permitted to use the "as-if" rule. That is,
> > > a compiler can emit whatever code it likes, as long as
> > > the results of a single-threaded execution appear just
> > > as if the compiler had followed all the relevant rules.
> > > To see this, compile with a high level of optimization
> > > and run the debugger on the resulting binary.
> >
> > That's okay for the single-CPU case. I don't think it covers the
> > multiple-CPU single-variable case correctly, though. If you don't use
> > READ_ONCE or WRITE_ONCE, isn't the compiler allowed to tear the loads
> > and stores? And won't that potentially cause the end result to be
> > different from what you would get if the code had appeared to execute
> > in order?
>
> Ah, good point, I need yet another qualifier. How about the following?
>
> b. Compilers are permitted to use the "as-if" rule. That is,
> a compiler can emit whatever code it likes for normal
> accesses, as long as the results of a single-threaded
> execution appear just as if the compiler had followed
> all the relevant rules. To see this, compile with a
> high level of optimization and run the debugger on the
> resulting binary.
>
> I added "for normal accesses", which excludes READ_ONCE(), WRITE_ONCE(),
> and atomics. This, in conjunction with the previously added
> "single-threaded execution" means that yes, the compiler is permitted
> to tear normal loads and stores. The reason is that a single-threaded
> run could not tell the difference. Interrupt handlers or multiple
> threads are required to detect load/store tearing.
>
> So, what am I still missing? ;-)

Well, you could explicitly mention that in the multi-thread case, this
means all accesses to the shared variable had better use READ_ONCE() or
WRITE_ONCE().

Alan

\
 
 \ /
  Last update: 2017-10-22 17:15    [W:0.076 / U:0.140 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site