2 futex questions

2 futex questions

[alfred]

1) There's a comment in sys_futex(...) that says pos_in_page must not be on a page boundary.
Could someone explain why?

2) How is this ever true:
	pos_in_page + sizeof(int) > PAGE_SIZE
when checking if pos_in_page is valid?


Thanks,
Alfred Landrum

Re: 2 futex questions

[Ingo Molnar]

On Tue, 24 Sep 2002 alfred@leakybucket.org wrote:

> 1) There's a comment in sys_futex(...) that says pos_in_page must not be
> on a page boundary. Could someone explain why?

what it wants to say: 'the futex word must not overflow into the next
page', ie. the futex word needs to be on a single page.

> 2) How is this ever true:
> 	pos_in_page + sizeof(int) > PAGE_SIZE
> when checking if pos_in_page is valid?

the full test is this:

        pos_in_page = ((unsigned long)uaddr) % PAGE_SIZE;

        if ((pos_in_page % __alignof__(int)) != 0
            || pos_in_page + sizeof(int) > PAGE_SIZE)
                return -EINVAL;

what it says: 'uaddr must be naturally aligned, and the word must be on a
single page'. In theory it's possible that __alignof__(int) !=
sizeof(int).

	Ingo

[patch] Re: 2 futex questions

[alfred]

On Wed, Sep 25, 2002 at 10:23:16AM +0200, Ingo Molnar wrote:
> 
> what it says: 'uaddr must be naturally aligned, and the word must be on a
> single page'. In theory it's possible that __alignof__(int) !=
> sizeof(int).
> 

Ok, would the following actually save some cycles then?

diff -u a/kernel/futex.c b/kernel/futex.c 
--- a/kernel/futex.c    Tue Sep 24 15:25:01 2002
+++ b/kernel/futex.c    Tue Sep 24 18:09:09 2002
@@ -321,9 +321,10 @@
 
        pos_in_page = ((unsigned long)uaddr) % PAGE_SIZE;
 
-       /* Must be "naturally" aligned, and not on page boundary. */
+       /* Must be "naturally" aligned, and must not cross a page boundary. */
        if ((pos_in_page % __alignof__(int)) != 0
-           || pos_in_page + sizeof(int) > PAGE_SIZE)
+           || ((sizeof(int) != __alignof__(int))
+                && (pos_in_page + sizeof(int) > PAGE_SIZE)))
                return -EINVAL;
 
        /* Simpler if it doesn't vanish underneath us. */


Alfred Landrum