-----Original Message-----
From: Esben Nielsen [mailto:nielsen.esben@gogglemail.com]
Sent: Tuesday, August 29, 2006 5:57 PM
To: Liu Dave-r63238
Cc: Li Yang-r58472; Esben Nielsen; Xupei Liang;

linuxppc-embedded@ozlabs.org

Subject: RE: atomic operations in user space
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On Tue, 29 Aug 2006, Liu Dave-r63238 wrote:
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2) These mutexes are based on futexes which requires atomic
operations in userspace. These are available on most

architectures.

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Look at

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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.

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There is only a system call if there is congestion - that is the whole

idea

behind the futex.
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And the
true atomic operation is in kernel not user space.

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"True" atomic operations are available in user space on most

architectures.

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Maybe
it's feasible
for other architectures to do atomic operations directly in
user space.
IMHO, not for powerpc.

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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.

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Are you meaning that we didn't do atomic operations directly in user
space
on powerpc platform ?

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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.
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This is by the way probably what the original post in this thread

wanted

in the first place!
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Esben
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-DAve