atomic operations in user space

[Esben Nielsen]

On Tue, 29 Aug 2006, Li Yang-r58472 wrote:

>> -----Original Message-----
>> From: linuxppc-embedded-bounces+leoli=freescale.com at ozlabs.org
>> [mailto:linuxppc-embedded-bounces+leoli=freescale.com at ozlabs.org] On
> Behalf Of
>> Xupei Liang
>> Sent: Tuesday, August 29, 2006 8:44 AM
>> To: linuxppc-embedded at ozlabs.org
>> Subject: RE: atomic operations in user space
>>
>> I think it is less expensive using atomic operation
>> sometimes in the user space, e.g. when updating a
>
> Atomic operations are working under strict restrictions.  Generally they
> won't work for user space.  Your best bet is to use semaphore instead.
>

Wrong.
1) Semaphores give priority inversions. Use a mutex with priority 
inheritance instead. This comes in 2.6.18.
2) These mutexes are based on futexes which requires atomic operations in 
userspace. These are available on most architectures. Look at the glibc code
in nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h forinstance. 
Use that and your PPC manual to implement your atomic operations.

Esben

>> counter. If this counter is to be updated by a lot of
>> processes, using semaphore can potentially cause a lot
>> of task switching.
>>
>> Regards,
>>
>> Terry Liang
>>
>>
>>> -----Original Message-----
>>> From: Brent Cook [mailto:bcook at bpointsys.com]
>>> Sent: Thursday, August 24, 2006 10:18 PM
>>> To: linuxppc-embedded at ozlabs.org
>>> Cc: Li Yang-r58472; Terry Liang
>>> Subject: Re: atomic operations in user space
>>>
>>> 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?
>>
>> Is there anything preventing semaphore to be used in
>> threads?
>>>
>>> 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
>>
>>
>>
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