atomic operations in user space

Thanks. Arnd. My main concern is whether the operations are really atomic as they are in the kernel space. I have read some discussion in another forum that on other platforms, even if you are able to compile the atomic_add(), atomic_set(), etc. from an user space application,  they don't guarantee to be atomic. Thanks.

Regards,

Terry Liang

Why do you need atomic operations in user land?  IPC will be sufficient
to deal with race conditions between processes.=20

Best Regards,
Leo

-----Original Message-----
From: linuxppc-embedded-bounces+leoli=3Dfreescale.com@ozlabs.org
[mailto:linuxppc-embedded-bounces+leoli=3Dfreescale.com@ozlabs.org] On

Behalf Of

Terry Liang
Sent: Thursday, August 24, 2006 3:04 AM
To: linuxppc-embedded@ozlabs.org
Subject: atomic operations in user space
=20
Thanks. Arnd. My main concern is whether the operations are really

atomic as they

are in the kernel space. I have read some discussion in another forum

that on other

platforms, even if you are able to compile the atomic_add(),

atomic_set(), etc.

from an user space application,  they don't guarantee to be atomic.

Thanks.

=20
Regards,
=20
Terry Liang

On Thursday 24 August 2006 05:39, Li Yang-r58472 wrote:

Why do you need atomic operations in user land?  IPC will be sufficient
to deal with race conditions between processes.

Best Regards,
Leo

What about multiple threads within a process updating a counter?

Of course, if you look at these functions in the kernel header, they're just 2 or 3 inline assembly calls - you could easily rewrite them. Google for 'PowerPC atomic increment' and grab one of the unencumbered implementations if you need to use it in a non-GPL program.

On the other hand, I see no license at the top of my /usr/include/asm-i386/atomic.h file at all, same for PowerPC - are Linux header files actually GPL or are they more like the glibc headers, with exceptions made for userspace programs?

The atomic operations on x86 were accidentally exported early on, so they have to hang around apparently for compatibility (there are some mailing list threads out there to this effect.) Currently, you just have to assume in Linux that if you include something from /usr/include/linux or asm that it will not necessarily be cross-version or cross-architecture compatible. Not every arch in Linux even has atomic operations of this nature, which I guess is the main reason why they are not exported in general.

 - Brent

quoted

-----Original Message-----
From: linuxppc-embedded-bounces+leoli=freescale.com@ozlabs.org
[mailto:linuxppc-embedded-bounces+leoli=freescale.com@ozlabs.org] On

Behalf Of

quoted

Terry Liang
Sent: Thursday, August 24, 2006 3:04 AM
To: linuxppc-embedded@ozlabs.org
Subject: atomic operations in user space

Thanks. Arnd. My main concern is whether the operations are really

atomic as they

quoted

are in the kernel space. I have read some discussion in another forum

that on other

quoted

platforms, even if you are able to compile the atomic_add(),

atomic_set(), etc.

quoted

from an user space application,  they don't guarantee to be atomic.

Thanks.

quoted

Regards,

Terry Liang

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-----Original Message-----
From: Brent Cook [mailto:bcook@bpointsys.com]
Sent: Thursday, August 24, 2006 10:18 PM
To: linuxppc-embedded@ozlabs.org
Cc: Li Yang-r58472; Terry Liang
Subject: Re: atomic operations in user space
=20
On Thursday 24 August 2006 05:39, Li Yang-r58472 wrote:
=20

quoted

Why do you need atomic operations in user land? IPC will be

sufficient

=20

quoted

to deal with race conditions between processes.

=20

quoted

=20

quoted

Best Regards,

=20

quoted

Leo

=20
What about multiple threads within a process updating a counter?

Is there anything preventing semaphore to be used in threads?

=20
Of course, if you look at these functions in the kernel header,

they're just 2 or

3 inline assembly calls - you could easily rewrite them. Google for

'PowerPC atomic

increment' and grab one of the unencumbered implementations if you

need to use it

in a non-GPL program.
=20
On the other hand, I see no license at the top of my

/usr/include/asm-i386/atomic.h

file at all, same for PowerPC - are Linux header files actually GPL or

are they

more like the glibc headers, with exceptions made for userspace

programs?

=20
The atomic operations on x86 were accidentally exported early on, so

they have to

hang around apparently for compatibility (there are some mailing list

threads out

there to this effect.) Currently, you just have to assume in Linux

that if you

include something from /usr/include/linux or asm that it will not

necessarily be

cross-version or cross-architecture compatible. Not every arch in

Linux even has

atomic operations of this nature, which I guess is the main reason why

they are

not exported in general.
=20
- Brent