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

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Message-Id: <20220131225250.409564-1-ndesaulniers@google.com>
Date:   Mon, 31 Jan 2022 14:52:47 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH] docs/memory-barriers.txt: volatile is not a barrier() substitute

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;
+
  (*) 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