compiler.h File Reference

Definition of memory barriers. More...

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Macros
#define	WRITE_ONCE(var, val)   (*((__volatile__ __typeof__(val) *)(&(var))) = (val))
	Assign the value to the given variable.
#define	READ_ONCE(var)   (*((__volatile__ __typeof__(var) *)(&(var))))
	Read the value from the given variable.

Detailed Description

Definition of memory barriers.

Generally speaking, memory barriers prevent the compiler from merging or refetching reads or writes. 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.

Follows an extract from LINUX KERNEL MEMORY BARRIERS by: David Howells, Paul E. McKenney, Will Deacon, Peter Zijlstra, available at: https://www.kernel.org/doc/Documentation/memory-barriers.txt

Consider the following abstract model of the system:

            :                :
+-------+   :   +--------+   :   +-------+
| CPU 1 |<----->| Memory |<----->| CPU 2 |
+-------+   :   +--------+   :   +-------+
    ^       :       ^        :       ^
    |       :       |        :       |
    |       :       v        :       |
    |       :   +--------+   :       |
    +---------->| Device |<----------+
            :   +--------+   :
            :                :

Each CPU executes a program that generates memory access operations. In the abstract CPU, memory operation ordering is very relaxed, and a CPU may actually perform the memory operations in any order it likes, provided program causality appears to be maintained. Similarly, the compiler may also arrange the instructions it emits in any order it likes, provided it doesn't affect the apparent operation of the program.

So in the above diagram, the effects of the memory operations performed by a CPU are perceived by the rest of the system as the operations cross the interface between the CPU and rest of the system (the dotted lines).

Copyright

(c) 2014-2024 This file is distributed under the MIT License. See LICENSE.md for details.