[ltt-dev] Userspace RCU library relicensed to LGPLv2.1

Steve Munroe sjmunroe at us.ibm.com
Thu May 14 11:17:56 EDT 2009

Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center

libc-alpha-owner at sourceware.org wrote on 05/14/2009 08:06:39 AM:

> * Jan Blunck (jblunck at suse.de) wrote:
> > On Wed, May 13, Mathieu Desnoyers wrote:
> >
> > > It currently supports x86 and powerpc. LGPL-compatible low-level
> > > primitive headers will be required for other architectures. Note that
> > > the build system is at best rudimentary at the moment.
> >
> > Is there a specific reason why the atomic_ops implementation was
> used instead
> > of the atomic builtins that come with GCC? IIRC, they are implemented
on all
> > architectures already.
> >
> Hi Jan,
> As said Evgeniy, there is the compiler version issue, but in this case
> there is more :
> If we look at
> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html
> The instruction closest to an xchg() instruction (to exchange a pointer
> in memory) is :
> "type __sync_lock_test_and_set (type *ptr, type value, ...)
>     This builtin, as described by Intel, is not a traditional
> test-and-set operation, but rather an atomic exchange operation. It
> writes value into *ptr, and returns the previous contents of *ptr.
It seems like either:

bool __sync_bool_compare_and_swap (type *ptr, type oldval type newval, ...)
type __sync_val_compare_and_swap (type *ptr, type oldval type newval, ...)
      These builtins perform an atomic compare and swap. That is, if the
      current value of *ptr is oldval, then write newval into *ptr.

      The “bool” version returns true if the comparison is successful and
      newval was written. The “val” version returns the contents of *ptr
      before the operation.

would do the trick with __sync_val_compare_and_swap and simple while loop.
Most of the time a single iteration is all that is required and on PowerPC
is the same loop you would need for xchg().

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