atomic operations in user space

[Li Yang-r58472]

> -----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.

> 
> >
> > 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