Re: [Xenomai-core] [3/9] Define more atomic operations in user-space

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>  > Gilles Chanteperdrix wrote:
>  > > On Fri, Apr 25, 2008 at 4:01 PM, Philippe Gerum <rpm@xenomai.org> wrote:
>  > >> Gilles Chanteperdrix wrote:
>  > >>  > On Fri, Apr 25, 2008 at 3:42 PM, Philippe Gerum <rpm@xenomai.org> wrote:
>  > >>  >> Gilles Chanteperdrix wrote:
>  > >>  >>  > On Fri, Apr 25, 2008 at 9:48 AM, Philippe Gerum <rpm@xenomai.org> wrote:
>  > >>  >>  >> Gilles Chanteperdrix wrote:
>  > >>  >>  >>  > This patch implements the _read, _set, and _cmpxchg operations on atomic_long_t
>  > >>  >>  >>  > and atomic_ptr_t in user-space in include/asm-generic/atomic.h which should be
>  > >>  >>  >>  > included at the end of include/asm-*/atomic.h after the definition of the same
>  > >>  >>  >>  > operations for the atomic_t type and atomic64_t type on 64 bits platforms.
>  > >>  >>  >>  >
>  > >>  >>  >>  > These operations are the basic operations used by user-space mutexes. Maybe we
>  > >>  >>  >>  > should add the xnarch_ prefix ?
>  > >>  >>  >>  >
>  > >>  >>  >>
>  > >>  >>  >>  Yes, but more generally, we should rework this to fit the existing atomic
>  > >>  >>  >>  support in include/asm-*/atomic.h, so that we don't end up with sideways to what
>  > >>  >>  >>  has been already designed to support set, get, xchg and the like, in both kernel
>  > >>  >>  >>  and userland context.
>  > >>  >>  >
>  > >>  >>  > That is not exactly sideways... Linux include/asm-generic/atomic.h
>  > >>  >>  > defines operations for atomic_long_t. This file adds two things:
>  > >>  >>  > - support for atomic_long_t in user-space (where we can not include
>  > >>  >>  > linux include/asm-generic/atomic.h)
>  > >>  >>  > - support for a new type atomic_ptr_t both to kernel-space and
>  > >>  >>  > user-space, the aim is to avoid all the casts that would take place if
>  > >>  >>  > we wanted to use atomic_long_t to store pointers.
>  > >>  >>  >
>  > >>  >>  > However for this file to work, it has to be included by asm-*/atomic.h
>  > >>  >>  > after the definition of atomic_t (and atomic64_t on 64 bits
>  > >>  >>  > platforms). So linux includes asm-generic/atomic.h at the end of
>  > >>  >>  > asm/atomic.h, I simply reproduced this scheme with Xenomai
>  > >>  >>  > include/asm-*/atomic.h.
>  > >>  >>  >
>  > >>  >>
>  > >>  >>  Focusing on user-space: 1) xnarch_atomic_xchg is meant to work on long types; 2)
>  > >>  >>  set, get routines are not defined in that scope. If the purpose is to define
>  > >>  >>  integer-type ops to handle pointer-type data atomically (i.e. intptr_t), then I
>  > >>  >>  would rather check whether we actually need non-long support at all in
>  > >>  >>  user-space. In case we don't, I would simply reply on the existing
>  > >>  >>  implementation of asm-*/atomic.h + the set / get extensions you provide.
>  > >>  >
>  > >>  > I use both atomic_t and atomic_ptr_t for the implementation of
>  > >>  > user-space mutexes. The problem is that I am constrained by the size
>  > >>  > of pthread_mutex_t, so the "control block read-write locks"
>  > >>  > implementation use atomic_t.
>  > >>  >
>  > >>
>  > >>  Ok, makes sense. Let's just fix the namespace then, so that we don't get the
>  > >>  feeling of having duplicate sets of operations.
>  > > 
>  > > That said, there is just enough room for replacing the atomic_t with
>  > > an atomic_long_t. So, we can make xnarch_atomic_t a long type. Have
>  > > you anything agains making an xnarch_atomic_ptr_t ?
>  > > 
>  > 
>  > No objection, just let us call it xnarch_atomic_intptr_t to ANSIfy this a bit
>  > more, and clearly state that we need this type to hold a pointer into an integer
>  > value, and operate atomically on it.
> 
> Ok. The current type defined in asm-arm/atomic.h is
> atomic_counter_t. Should I rename this xnarch_atomic_t ?
>

Fine with me.

> I ask this now, because I am going to do search and replaces of atomic_
> all over my changes, so, I prefer to know before.
> 


-- 
Philippe.