Messages in this thread | | | Subject | Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo | From | Akira Yokosawa <> | Date | Thu, 7 Mar 2019 00:46:05 +0900 |
| |
On Tue, 26 Feb 2019 16:04:50 +0100, Peter Zijlstra wrote: > On Tue, Feb 26, 2019 at 06:28:45AM -0800, Paul E. McKenney wrote: > >> Yes, this all is a bit on the insane side from a kernel viewpoint. >> But the paper you found does not impose this; it has instead been there >> for about 20 years, back before C and C++ admitted to the existence >> of concurrency. But of course compilers are getting more aggressive, >> and yes, some of the problems show up in single-threaded code. > > But that paper is from last year!! It has Peter Sewell on, I'm sure he's > heard of concurrency. > >> The usual response is "then cast the pointers to intptr_t!" but of >> course that breaks type checking. > > I tried laundering the pointer through intptr_t, but I can't seem to > unbreak it. > > > root@ivb-ep:~/tmp# gcc-8 -O2 -fno-strict-aliasing -o ptr ptr.c ; ./ptr > p=0x55aacdc80034 q=0x55aacdc80034 > x=1 y=2 *p=11 *q=2 > root@ivb-ep:~/tmp# cat ptr.c > #include <stdio.h> > #include <string.h> > #include <stdint.h> > int y = 2, x = 1; > int main (int argc, char **argv) { > intptr_t P = (intptr_t)&x; > intptr_t Q = (intptr_t)&y; > P += sizeof(int); > int *q = &y; > printf("p=%p q=%p\n", (int*)P, (int*)Q); > if (P == Q) { > int *p = (int *)P; > *p = 11; > printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q); > } > } >
So, I'm looking at the macro RELOC_HIDE() defined in include/linux/compiler-gcc.h.
It says:
-------- /* * This macro obfuscates arithmetic on a variable address so that gcc * shouldn't recognize the original var, and make assumptions about it. * * This is needed because the C standard makes it undefined to do * pointer arithmetic on "objects" outside their boundaries and the * gcc optimizers assume this is the case. In particular they * assume such arithmetic does not wrap. * [...] */ #define RELOC_HIDE(ptr, off) \ ({ \ unsigned long __ptr; \ __asm__ ("" : "=r"(__ptr) : "0"(ptr)); \ (typeof(ptr)) (__ptr + (off)); \ }) --------
Looks like this macro has existed ever since the origin of Linus' git repo.
And the optimization "bug" discussed in this thread can be suppressed by this macro.
For example,
$ gcc -O2 -o reloc_hide reloc_hide.c; ./reloc_hide x=1 y=11 *p=11 *q=11 $ cat reloc_hide.c #include <stdio.h> #include <stdint.h>
#define RELOC_HIDE(ptr, off) \ ({ \ uintptr_t __ptr; \ __asm__ ("" : "=r"(__ptr) : "0"(ptr)); \ (typeof(ptr)) (__ptr + (off)); \ })
int y = 2, x = 1; int main (int argc, char **argv) { int *p = RELOC_HIDE(&x, sizeof(*p)); int *q = RELOC_HIDE(&y, 0); if (p == q) { *p = 11; printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q); } }
Note that "uintptr_t" is used in this version of RELOC_HIDE() for user-land code.
Am I the only one who was not aware of this gcc-specific macro?
Thanks, Akira
| |