Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute

[Ard Biesheuvel]

On Tue, 1 Feb 2022 at 20:40, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Tue, Feb 1, 2022 at 1:32 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> > On Mon, 31 Jan 2022 at 23:53, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > >
> > > Add text to memory-barriers.txt and deprecated.rst to denote that
> > > volatile-qualifying an asm statement is not a substitute for either a
> > > compiler barrier (``barrier();``) or a clobber list.
> > >
> > > This way we can point to this in code that strengthens existing
> > > volatile-qualified asm statements to use a compiler barrier.
> > >
> > > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > ---
> > > Example: https://godbolt.org/z/8PW549zz9
> > >
> > >  Documentation/memory-barriers.txt    | 24 ++++++++++++++++++++++++
> > >  Documentation/process/deprecated.rst | 17 +++++++++++++++++
> > >  2 files changed, 41 insertions(+)
> > >
> > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > > index b12df9137e1c..f3908c0812da 100644
> > > --- a/Documentation/memory-barriers.txt
> > > +++ b/Documentation/memory-barriers.txt
> > > @@ -1726,6 +1726,30 @@ of optimizations:
> > >       respect the order in which the READ_ONCE()s and WRITE_ONCE()s occur,
> > >       though the CPU of course need not do so.
> > >
> > > + (*) Similarly, the compiler is within its rights to reorder instructions
> >
> > Similar to what? Was this intended to be the second bullet point
> > rather than the first?
>
> Similar to the previous bullet point. This isn't the first use of
> `Similarly, ` in this document.
>

Ah right, I misread the context and thought you were inserting this
bullet point at the start. Sorry for the noise.

> >
> > > +     around an asm statement so long as clobbers are not violated. For example,
> > > +
> > > +       asm volatile ("");
> > > +       flag = true;
> > > +
> > > +     May be modified by the compiler to:
> > > +
> > > +       flag = true;
> > > +       asm volatile ("");
> > > +
> > > +     Marking an asm statement as volatile is not a substitute for barrier(),
> > > +     and is implicit for asm goto statements and asm statements that do not
> > > +     have outputs (like the above example). Prefer either:
> > > +
> > > +       asm ("":::"memory");
> > > +       flag = true;
> > > +
> > > +     Or:
> > > +
> > > +       asm ("");
> > > +       barrier();
> > > +       flag = true;
> > > +
> >
> > I would expect the memory clobber to only hazard against the
> > assignment of flag if it results in a store, but looking at your
> > Godbolt example, this appears to apply even if flag is kept in a
> > register.
> >
> > Is that behavior documented/codified anywhere? Or are we relying on
> > compiler implementation details here?
>
> https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
> "Note that the compiler can move even volatile asm instructions
> relative to other code, including across jump instructions."
>

That doesn't really answer my question. We are documenting here that
asm volatile does not prevent reordering but non-volatile asm with a
"memory" clobber does, and even prevents reordering of instructions
that do not modify memory to begin with.

Why is it justified to rely on this undocumented behavior?


> >
> >
> > >   (*) The compiler is within its rights to invent stores to a variable,
> > >       as in the following example:
> > >
> > > diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> > > index 388cb19f5dbb..432816e2f79e 100644
> > > --- a/Documentation/process/deprecated.rst
> > > +++ b/Documentation/process/deprecated.rst
> > > @@ -329,3 +329,20 @@ struct_size() and flex_array_size() helpers::
> > >          instance->count = count;
> > >
> > >          memcpy(instance->items, source, flex_array_size(instance, items, instance->count));
> > > +
> > > +Volatile Qualified asm Statements
> > > +=================================
> > > +
> > > +According to `the GCC docs on inline asm
> > > +https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile`_:
> > > +
> > > +  asm statements that have no output operands and asm goto statements,
> > > +  are implicitly volatile.
> > > +
> > > +For many uses of asm statements, that means adding a volatile qualifier won't
> > > +hurt (making the implicit explicit), but it will not strengthen the semantics
> > > +for such cases where it would have been implied. Care should be taken not to
> > > +confuse ``volatile`` with the kernel's ``barrier()`` macro or an explicit
> > > +clobber list. See [memory-barriers]_ for more info on ``barrier()``.
> > > +
> > > +.. [memory-barriers] Documentation/memory-barriers.txt
> > > --
> > > 2.35.0.rc2.247.g8bbb082509-goog
> > >
>
>
>
> --
> Thanks,
> ~Nick Desaulniers