Re: [LTP] mmap syscall problem

Valdis.Kletnieks
2009-07-07T16:25:01Z

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On Mon, 06 Jul 2009 16:16:11 +0200, Arnd Bergmann said:

> What happens in a simple user space program that mmaps the same
> page to two addresses? Something like
>
> #include <sys/mman.h>
> #include <fcntl.h>
> int main(void)
> {
> int fd = open("existing-4k-file", O_RDWR);
> char *p1 = mmap(0, 4096, PROT_READ | PROT_WRITE,
> MAP_SHARED, fd, 0);
> char *p2 = mmap(p1 + 4096, 4096, PROT_READ | PROT_WRITE,
> MAP_SHARED | MAP_FIXED, fd, 0);
>
> *p1 = 0xaa; *p2 = 0x55;
>
> return *p1; /* returns 0xaa if broken, 0x55 if correct */
> }

Strictly speaking, doesn't that need a barrier of some sort between the two
assignments? I mean, how is gcc supposed to intuit that p1 and p2 alias
each other and it needs to reload *p1 before returning that value?

(And in more complicated code, I could even see the compiler re-ordering those
two assignments, so the in-file value is different as well as the return; value)

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