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

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Feb 1, 2022 at 1:32 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Mon, 31 Jan 2022 at 23:53, Nick Desaulniers <ndesaulniers@google.com> 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@chromium.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > 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.

>
> > +     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."

>
>
> >   (*) 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