Re: [PATCH 9/9] kernel: tighten rules for ACCESS ONCE
From: Christian Borntraeger
Date: Thu Dec 04 2014 - 04:28:36 EST
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 This comment is obsolete in the same way as that of READ_ONCE() and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(). ;-)
* 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.
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