[tip:locking/core] locking/atomics: Update comment about READ_ONCE() and structures

[tip-bot for Konrad Rzeszutek Wilk <tipbot@zytor.com>]

Commit-ID:  fed0764fafd8e2e629a033c0f7df4106b0dcb7f0
Gitweb:     http://git.kernel.org/tip/fed0764fafd8e2e629a033c0f7df4106b0dcb7f0
Author:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
AuthorDate: Mon, 25 Jan 2016 16:33:20 -0500
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 9 Feb 2016 14:50:16 +0100

locking/atomics: Update comment about READ_ONCE() and structures

The comment is out of data. Also point out the performance drawback
of the barrier();__builtin_memcpy(); barrier() followed by another
copy from stack (__u) to lvalue;

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1453757600-11441-1-git-send-email-konrad.wilk@oracle.com
[ Made it a bit more readable. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/compiler.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 00b042c..4291592 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -263,8 +263,9 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * In contrast to ACCESS_ONCE these two macros will also work on aggregate
  * data types like structs or unions. If the size of the accessed data
  * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
- * READ_ONCE() and WRITE_ONCE()  will fall back to memcpy and print a
- * compile-time warning.
+ * READ_ONCE() and WRITE_ONCE() will fall back to memcpy(). There's at
+ * least two memcpy()s: one for the __builtin_memcpy() and then one for
+ * the macro doing the copy of variable - '__u' allocated on the stack.
  *
  * Their two major use cases are: (1) Mediating communication between
  * process-level code and irq/NMI handlers, all running on the same CPU,