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Ensure that the data in ringbuff is accessed in the right order

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This commit is contained in:
Moritz 'Morty' Strübe 2014-11-28 12:22:24 +01:00
parent 552408b241
commit d68cbb2980
2 changed files with 32 additions and 4 deletions
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25
core/lib/ringbuf.c
View file

@ -38,6 +38,7 @@
*/
#include "lib/ringbuf.h"
#include <sys/cc.h>
/*---------------------------------------------------------------------------*/
void
ringbuf_init(struct ringbuf *r, uint8_t *dataptr, uint8_t size)
@ -63,8 +64,15 @@ ringbuf_put(struct ringbuf *r, uint8_t c)
if(((r->put_ptr - r->get_ptr) & r->mask) == r->mask) {
return 0;
}
r->data[r->put_ptr] = c;
r->put_ptr = (r->put_ptr + 1) & r->mask;
/*
* CC_ACCESS_NOW is used because the compiler is allowed to reorder
* the access to non-volatile variables.
* In this case a reader might read from the moved index/ptr before
* its value (c) is written. Reordering makes little sense, but
* better safe than sorry.
*/
CC_ACCESS_NOW(uint8_t, r->data[r->put_ptr]) = c;
CC_ACCESS_NOW(uint8_t, r->put_ptr) = (r->put_ptr + 1) & r->mask;
return 1;
}
/*---------------------------------------------------------------------------*/
@ -84,8 +92,17 @@ ringbuf_get(struct ringbuf *r)
most platforms, but C does not guarantee this.
*/
if(((r->put_ptr - r->get_ptr) & r->mask) > 0) {
c = r->data[r->get_ptr];
r->get_ptr = (r->get_ptr + 1) & r->mask;
/*
* CC_ACCESS_NOW is used because the compiler is allowed to reorder
* the access to non-volatile variables.
* In this case the memory might be freed and overwritten by
* increasing get_ptr before the value was copied to c.
* Opposed to the put-operation this would even make sense,
* because the register used for mask can be reused to save c
* (on some architectures).
*/
c = CC_ACCESS_NOW(uint8_t, r->data[r->get_ptr]);
CC_ACCESS_NOW(uint8_t, r->get_ptr) = (r->get_ptr + 1) & r->mask;
return c;
} else {
return -1;

11
core/sys/cc.h
View file

@ -123,6 +123,17 @@
#define CC_NO_VA_ARGS CC_CONF_VA_ARGS
#endif
/** \def CC_ACCESS_NOW(x)
* This macro ensures that the access to a non-volatile variable can
* not be reordered or optimized by the compiler.
* See also https://lwn.net/Articles/508991/ - In Linux the macro is
* called ACCESS_ONCE
* The type must be passed, because the typeof-operator is a gcc
* extension
*/
#define CC_ACCESS_NOW(type, variable) (*(volatile type *)&(variable))
#ifndef NULL
#define NULL 0
#endif /* NULL */