atomic operations in user space

[Li Yang-r58472]

> -----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.

> 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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