Re: [PATCH tip/core/rcu 01/14] rcu: Protect rcu_boost() lockless accesses with ACCESS_ONCE()

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 12.01.2015 um 23:12 schrieb Paul E. McKenney:
> On Mon, Jan 12, 2015 at 09:59:57AM +0100, Peter Zijlstra wrote:
>> On Fri, Jan 09, 2015 at 10:58:50PM +0100, Christian Borntraeger wrote:
>>> Am 09.01.2015 um 14:56 schrieb Peter Zijlstra:
>>>> On Fri, Jan 09, 2015 at 05:49:54AM -0800, Paul E. McKenney wrote:
>>>>>> That reminds me, I think the new conversion for stores will most likely
>>>>>> introduce silly arg bugs:
>>>>>>
>>>>>> -       ACCESS_ONCE(a) = b;
>>>>>> +       ASSIGN_ONCE(b, a);
>>>>>
>>>>> I was planning to do mine by hand for this sort of reason.
>>>>>
>>>>> Or are you thinking of something more subtle than the case where
>>>>> "b" is an unparenthesized comma-separated expression?
>>>>
>>>> I think he's revering to the wrong way around-ness of the thing.
>>>>
>>>> Its a bit of a mixed bag on assignments, but for instance
>>>> rcu_assign_pointer() takes them the right way around, as does
>>>> atomic_set().
>>>>
>>>> So yes, I think the ASSIGN_ONCE() thing got the arguments the wrong way
>>>> around.
>>>>
>>>> We could maybe still change it, before its in too long ?
>>>
>>> Linus initial proposal was inspired by put_user model which is (val,
>>> ptr) and I took that. 
>>
>> Yeah, like I said, its a bit of a mixed bag. We've got plenty examples
>> of the wrong way around.
>>
>>> As my focus was on avoiding the volatile bug,
>>> all my current conversions are READ_ONCE as no potential ASSIGN_ONCE
>>> user was done on a non-scalar type, so I have no first hand
>>> experience. 
>>
>> So the implication there is that we'd preserve ACCESS_ONCE() for use on
>> scalar types. I don't think we should do that, I think we should just
>> en-mass convert to {READ,WRITE}/{LOAD,STORE}_ONCE() and kill off
>> ACCESS_ONCE().
> 
> Yep.  For one thing, the proposed replacements work much better with
> C11 than does ACCESS_ONCE().

As we agreed there is no perfect interface regarding val,x vs. x,val.
But it seems that there is some consensus that I should push something like the following (still whitespace damaged) to Linus for 3.19?
Peter, Davidlohr, Paul (maybe Linus) can you ACK/NACK?


Subject: Change ASSIGN_ONCE(val, x) to WRITE_ONCE(x, val)

Feedback has shown that WRITE_ONCE(x, val) is easier to use than ASSIGN_ONCE(val,x).
There are no in-tree users yet, so lets change it.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>


diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 84734a7..38865c7 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -215,7 +215,7 @@ static __always_inline void __read_once_size(volatile void *p, void *res, int si
        }
 }
 
-static __always_inline void __assign_once_size(volatile void *p, void *res, int size)
+static __always_inline void __write_once_size(volatile void *p, void *res, int size)
 {
        switch (size) {
        case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
@@ -235,15 +235,15 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
 /*
  * Prevent the compiler from merging or refetching reads or writes. The
  * compiler is also forbidden from reordering successive instances of
- * READ_ONCE, ASSIGN_ONCE and ACCESS_ONCE (see below), but only when the
+ * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
  * compiler is aware of some particular ordering.  One way to make the
  * compiler aware of ordering is to put the two invocations of READ_ONCE,
- * ASSIGN_ONCE or ACCESS_ONCE() in different C statements.
+ * WRITE_ONCE or ACCESS_ONCE() in different C statements.
  *
  * 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 ASSIGN_ONCE()  will fall back to memcpy and print a
+ * READ_ONCE() and WRITE_ONCE()  will fall back to memcpy and print a
  * compile-time warning.
  *
  * Their two major use cases are: (1) Mediating communication between
@@ -257,8 +257,8 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
 #define READ_ONCE(x) \
        ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })
 
-#define ASSIGN_ONCE(val, x) \
-       ({ typeof(x) __val; __val = val; __assign_once_size(&x, &__val, sizeof(__val)); __val; })
+#define WRITE_ONCE(x, val) \
+       ({ typeof(x) __val; __val = val; __write_once_size(&x, &__val, sizeof(__val)); __val; })
 
 #endif /* __KERNEL__ */
 
@@ -458,7 +458,7 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
  * with an explicit memory barrier or atomic instruction that provides the
  * required ordering.
  *
- * If possible use READ_ONCE/ASSIGN_ONCE instead.
+ * If possible use READ_ONCE/WRITE_ONCE instead.
  */
 #define __ACCESS_ONCE(x) ({ \
         __maybe_unused typeof(x) __var = (typeof(x)) 0;