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

[Nick Desaulniers <ndesaulniers@google.com>]

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
+     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