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Message-ID: <CAKwvOdnkGfeBBE2NW_FKSzmZSjCJXc2801qvvOuyu+JL+m+VZQ@mail.gmail.com>
Date: Tue, 1 Feb 2022 11:40:29 -0800
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Jonathan Corbet <corbet@....net>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev,
Kees Cook <keescook@...omium.org>,
Alan Stern <stern@...land.harvard.edu>,
Andrea Parri <parri.andrea@...il.com>,
Will Deacon <will@...nel.org>,
Boqun Feng <boqun.feng@...il.com>,
Nicholas Piggin <npiggin@...il.com>,
David Howells <dhowells@...hat.com>,
Jade Alglave <j.alglave@....ac.uk>,
Luc Maranget <luc.maranget@...ia.fr>,
"Paul E. McKenney" <paulmck@...nel.org>,
Akira Yokosawa <akiyks@...il.com>,
Daniel Lustig <dlustig@...dia.com>,
Joel Fernandes <joel@...lfernandes.org>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Len Baker <len.baker@....com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>,
Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute
On Tue, Feb 1, 2022 at 1:32 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Mon, 31 Jan 2022 at 23:53, Nick Desaulniers <ndesaulniers@...gle.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@...omium.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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
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