[PATCH] compiler.h: Clarify comment about the need for barrier_data()

[Arvind Sankar <nivedita@alum.mit.edu>]

Be clear about @ptr vs the variable that @ptr points to, and add some
more details as to why the special barrier_data() macro is required.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 include/linux/compiler.h | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 93035d7fee0d..d8cee7c8968d 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -86,17 +86,28 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 #ifndef barrier_data
 /*
- * This version is i.e. to prevent dead stores elimination on @ptr
- * where gcc and llvm may behave differently when otherwise using
- * normal barrier(): while gcc behavior gets along with a normal
- * barrier(), llvm needs an explicit input variable to be assumed
- * clobbered. The issue is as follows: while the inline asm might
- * access any memory it wants, the compiler could have fit all of
- * @ptr into memory registers instead, and since @ptr never escaped
- * from that, it proved that the inline asm wasn't touching any of
- * it. This version works well with both compilers, i.e. we're telling
- * the compiler that the inline asm absolutely may see the contents
- * of @ptr. See also: https://llvm.org/bugs/show_bug.cgi?id=15495
+ * This version is to prevent dead stores elimination on @ptr where gcc and
+ * llvm may behave differently when otherwise using normal barrier(): while gcc
+ * behavior gets along with a normal barrier(), llvm needs an explicit input
+ * variable to be assumed clobbered.
+ *
+ * Its primary use is in implementing memzero_explicit(), which is used for
+ * clearing temporary data that may contain secrets.
+ *
+ * The issue is as follows: while the inline asm might access any memory it
+ * wants, the compiler could have fit all of the variable that @ptr points to
+ * into registers instead, and if @ptr never escaped from the function, it
+ * proved that the inline asm wasn't touching any of it. gcc only eliminates
+ * dead stores if the variable was actually allocated in registers, but llvm
+ * reasons that the variable _could_ have been in registers, so the inline asm
+ * can't reliably access it anyway, and eliminates dead stores even if the
+ * variable is actually in memory.
+ *
+ * This version works well with both compilers, i.e. we're telling the compiler
+ * that the inline asm absolutely may see the contents of the variable pointed
+ * to by @ptr.
+ *
+ * See also: https://llvm.org/bugs/show_bug.cgi?id=15495#c5
  */
 # define barrier_data(ptr) __asm__ __volatile__("": :"r"(ptr) :"memory")
 #endif
-- 
2.26.2