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

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-----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

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Subject: RE: atomic operations in user space



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.

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

idea

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behind the futex.

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

"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.

It is available for PowerPC, but not in POWER and POWER2

instructionsets

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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

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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.

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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

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in the first place!

Esben

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-DAve

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