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authorKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>2016-01-25 16:33:20 -0500
committerIngo Molnar <mingo@kernel.org>2016-02-09 14:50:16 +0100
commitfed0764fafd8e2e629a033c0f7df4106b0dcb7f0 (patch)
tree11877084d79ccadb090b726cb280bb76e72ca4aa /include/linux/compiler.h
parent06bea3dbfe6a4c333c4333362c46bdf4d9e43504 (diff)
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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>
Diffstat (limited to 'include/linux/compiler.h')
-rw-r--r--include/linux/compiler.h5
1 files 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,
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