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SubjectRe: Change in functionality of futex() system call.
Hi Eric,

I'm finally getting time to review this in depth and try to help Shawn
get his fix upstream. Trying to make sure I have all the facets of this
straight in my head... or on paper at least ;-)

On 06/06/2011 11:27 AM, Eric Dumazet wrote:
> Le lundi 06 juin 2011 à 20:23 +0200, Peter Zijlstra a écrit :
>
>>
>> That's really not the point, what do we do when the COW happens during
>> the FUTEX_WAIT? At that point the process vaddr changes mapping and we
>> cannot continue the wait on the old page, since that would expose
>> invisible information, nor can we switch to the new page since we queued
>> on the old page.
>>
>> Therefore we have to force the COW and queue on the private copy, it
>> really is the only semi sane semantic.
>
> The point is we dont necessarly have to COW the page. If you attempt
> this COW, you shoot on user that did not expect to have a COW.
>
> Take this program : COW is not allowed, still this worked on 2.6.18 (it
> waits until another process change the value in file and call
> futex_wait())
>
> Using PROT_READ | PROT_WRITE instead of PROT_READ was OK too.
>
> (If we use PROT_READ | PROT_WRITE, then after your patch, program doesnt
> work anymore since this process gets a private page after your hidden
> COW : It'll wait forever)


As I understand MMAP(2), this is working due to undefined behavior as
Stephen pointed out earlier:

"It is unspecified whether changes made to the file after the mmap()
call are visible in the mapped region."

I don't think we are under any obligation to keep that working.

--
Darren

>
> #include <errno.h>
> #include <fcntl.h>
> #include <stdint.h>
> typedef uint32_t u32; // for futex.h
> #include <linux/futex.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <unistd.h>
>
>
> int main(int argc, char *argv[]) {
> int fd, *futex, rc, val = 42;
>
> fd = open("/tmp/futex_test", O_RDWR|O_CREAT, 0644);
> write(fd, &val, 4);
> futex = (int *)mmap(0, sizeof(int), PROT_READ, MAP_PRIVATE, fd, 0);
> rc = syscall(SYS_futex, futex, FUTEX_WAIT, val, 0, 0, 0);
> printf("rc=%d errno=%d\n", rc, errno);
> }
>
>
>
> --
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--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
--
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