Commit fed0764f authored Jan 25, 2016 by Konrad Rzeszutek Wilk Committed by Ingo Molnar Feb 09, 2016

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>

parent 06bea3db

include/linux/compiler.h

+3 −2

Original line number	Diff line number	Diff line
		@@ -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,