Re: [PATCH resend] linux/compiler.h: fix stale comment on {READ,WRITE}_ONCE

[Andrew Morton]

On Thu, 10 Mar 2016 01:16:57 +0100 Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote:

> Commit 7bd3e239d6c6 ("locking: Remove atomicy checks from
> {READ,WRITE}_ONCE") removed said warning.
> 
> Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> ---
> Andrew, can you pick this up please?
> 
>  include/linux/compiler.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 48f5aab117ae..062b6ad95276 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -263,8 +263,7 @@ 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.
>   *
>   * Their two major use cases are: (1) Mediating communication between
>   * process-level code and irq/NMI handlers, all running on the same CPU,

Conflicts with Konrad's patch in linux-next (from an IngoTree), below.

I'll drop your patch - Konrad covers the same info and more.

(grammar nazi: s/There's/There are/)

commit fed0764fafd8e2e629a033c0f7df4106b0dcb7f0
Author:     Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
AuthorDate: Mon Jan 25 16:33:20 2016 -0500
Commit:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue Feb 9 14:50:16 2016 +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@xxxxxxxxxx>
    Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
    Cc: Borislav Petkov <bp@xxxxxxxxx>
    Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
    Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
    Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
    Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
    Link: http://lkml.kernel.org/r/1453757600-11441-1-git-send-email-konrad.wilk@xxxxxxxxxx
    [ Made it a bit more readable. ]
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

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,