Re: [PATCH 9/9] kernel: tighten rules for ACCESS ONCE

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

Am 04.12.2014 um 01:16 schrieb Paul E. McKenney:
>>   * merging, or refetching absolutely anything at any time.  Its main intended
>> >   * use is to mediate communication between process-level code and irq/NMI
>> >   * handlers, all running on the same CPU.
> This comment is obsolete in the same way as that of READ_ONCE() and
> ASSIGN_ONCE(), but probably more to the point to just get rid of
> ACCESS_ONCE().  ;-)
> 
>> > 

Its now 

/*
 * Prevent the compiler from merging or refetching accesses.  The compiler
 * is also forbidden from reordering successive instances of ACCESS_ONCE(),
 * 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
 * ACCESS_ONCE() in different C statements.
 *
 * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
 * on a union member will work as long as the size of the member matches the
 * size of the union and the size is smaller than word size.
 *
 * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
 * between process-level code and irq/NMI handlers, all running on the same CPU,
 * and (2) Ensuring that the compiler does not  fold, spindle, or otherwise
 * mutilate accesses that either do not require ordering or that interact
 * with an explicit memory barrier or atomic instruction that provides the
 * required ordering.
 *
 * If possible use READ_ONCE/ASSIGN_ONCE instead.
 */