atomic operations in user space

[Esben Nielsen]

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

>
>> -----Original Message-----
>> From: Esben Nielsen [mailto:nielsen.esben at gogglemail.com]
>> Sent: Tuesday, August 29, 2006 5:57 PM
>> To: Liu Dave-r63238
>> Cc: Li Yang-r58472; Esben Nielsen; Xupei Liang;
> linuxppc-embedded at ozlabs.org
>> Subject: RE: atomic operations in user space
>>
>>
>>
>> On Tue, 29 Aug 2006, Liu Dave-r63238 wrote:
>>
>>>>> 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 for instance.
>>>>> Use that and your PPC manual to implement your atomic operations.
>>>>
>>>> No matter semaphore or futex, it uses system calls to kernel.
>>
>> There is only a system call if there is congestion - that is the whole
> idea
>> behind the futex.
>>
>>>> And the
>>>> true atomic operation is in kernel not user space.
>>
>> "True" atomic operations are available in user space on most
> architectures.
>>
>>>> Maybe
>>>> it's feasible
>>>> for other architectures to do atomic operations directly in
>>>> user space.
>>>> IMHO, not for powerpc.
>>
>> It is available for PowerPC, but not in POWER and POWER2
> instructionsets
>> according to http://www.nersc.gov/vendor_docs/ibm/asm/lwarx.htm#idx607
>> It is the same in the ARM world: Atomic instructions was introduced in
>> ARMv6 I believe. Older ARM processors don't have them.
>
> Yes, I do know there are lwarx and stwrx instructions.  But there is
> only one reservation per CPU and reservation can be re-established with
> no difference.
> So there are possibilities reservation is broken and reserved again in
> one atomic block.
>
> Task A			Task B
> lwarx
> 				......
> 				lwarx
> 				stwrx
>
> 				.....
> 				.....
> 				lwarx
> stwrx
> .....
> 				stwrx
>
> The addresses of above operations are the same.
>
> In this case Thread A thinks that it is atomic as it holds the same
> reservation, but it is actually broken.  Such control flow can't be
> prevented in user space.
>

So you are saying that futexes on powerpc are broken?

Esben

>>
>>>
>>> Are you meaning that we didn't do atomic operations directly in user
>>> space
>>> on powerpc platform ?
>>>
>>
>> Well, that is not the conclusion I get either when reading the glibc
> code.
>> Try to look at glibc-2.3.5/sysdeps/powerpc/bits/atomic.h.
>>
>> This is by the way probably what the original post in this thread
> wanted
>> in the first place!
>>
>> Esben
>>
>>
>>> -DAve
>
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