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