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

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Message-ID: <202201311550.31EF589B2@keescook>
Date:   Mon, 31 Jan 2022 15:53:23 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev,
        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@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] docs/memory-barriers.txt: volatile is not a barrier()
 substitute

On Mon, Jan 31, 2022 at 02:52:47PM -0800, Nick Desaulniers 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
> +     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 like this!

>   (*) 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
> +=================================

I would open with an example, like:

Instead of::

	volatile asm("...");

just use::

	asm("...");


> +
> +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.

Does this mean "volatile" _is_ needed when there are operands, etc?

> +
> +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
> 

-- 
Kees Cook