why mprotect(PROT_SEM) ?

Hello list!

In my quest to remove (or at least handle the size dynamically) of
/dev/shm/ngpt I just read about sys_futex.c/sys_futex_region() and found, it
calls in the end only
	mprotect(area, size, PROT_READ|PROT_WRITE|PROT_SEM);
The read and write protection should be OK anyway (the file /dev/shm/ngpt
itself has access rights 666) so the only interesting point here is PROT_SEM.
This macro came new into the kernel with the futex patch. It is defined in
asm-(IBM-architectures)/mman.h as

#define PROT_SEM 0x8 /* page may be used for atomic ops */

and used only once: in mm/mprotect.c/sys_mprotect() this new flag is also
allowed but nobody does anything with it.

So what can we learn from this? Is this mprotect() call necessary at all? May
it be, that other architectures may need to prepare something more to allow
atomic operations in user pages?

Bye, Uwe

-- 
Dr. Uwe Girlich              email: [email protected]
Philosys Software GmbH       www: www.philosys.de
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