Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc

[Grant Grundler <grundler@parisc-linux.org>]

[davem: patch for you at the bottom to Documentation/atomic_ops.txt ]

On Tue, Aug 28, 2007 at 02:38:35PM -0400, Mathieu Desnoyers wrote:
> * Grant Grundler (grundler@parisc-linux.org) wrote:
> > On Tue, Aug 28, 2007 at 07:50:18AM -0400, Mathieu Desnoyers wrote:
...
> > > So I don't expect to come with an "upper bound" about where it can be
> > > used...
> > 
> > Ok...so I'll try to find one in 2.6.22.5:
> > grundler <1855>find -name \*.c | xargs fgrep DEFINE_PER_CPU | fgrep atomic_t
> > ./arch/s390/kernel/time.c:static DEFINE_PER_CPU(atomic_t, etr_sync_word);
> > grundler <1856>find -name \*.c | xargs fgrep DEFINE_PER_CPU | fgrep local_t
> > ./arch/x86_64/kernel/nmi.c:static DEFINE_PER_CPU(local_t, alert_counter);
> > 
> > uhm, I was expecting more than that.  Maybe there is some other systemic
> > problem with how PER_CPU stuff is used/declared?
> 
> the local ops has just been standardized in 2.6.22 though a patchset I
> did. I would not expect the code to start using them this quickly. Or
> maybe is it just that I am doing a terrible marketing job ;)

Yeah, I didn't expect many users of local_t.

The search for atomic_t usage in DEFINE_PER_CPU was an attempt to
find other potential candidates which could be local_t.
Is there any other programmatic way we could look for code which
could (or should) be using local_t?

> > In any case, some references to LTT usage would be quite helpful.
> > E.g. a list of file and variable names at the end of local_ops.txt file.
> > 
> 
> LTT is not mainlined (yet!) ;)

Ok...probably not such a good example then. :)

...
> > Sorry...the quoted text doesn't answer my question. It's a definition
> > of semantics, not an explanation of the "mechanics".
> > 
> > I want to know what happens when (if?) an interrupt occurs in the
> > middle of a read/modify/write sequence that isn't prefixed with LOCK
> > (or something similar for other arches like "store locked conditional" ops).
> > 
> > Stating the semantics is a good thing - but not a substitution for
> > describing how it works for a given architecture. Either in the code
> > or in local_ops.txt. Otherwise people like me won't use it because
> > we don't believe that (or understand how) it really works.
> > 
> 
> Quoting Intel 64 and IA-32 Architectures Software Developer's Manual
> 
> 3.2 Instructions
> LOCK - Assert LOCK# Signal Prefix
...

I've read this before and understand how LOCK works. This isn't helpful
since I want a description of the behavior without LOCK.

> And if we take a look at some of the atomic primitives which are used in
> i386 local.h:
> 
> add (for inc/dec/add/sub)
> xadd
> cmpxchg
> 
> All these instructions, just like any other, can be interrupted by an
> external interrupt or cause a trap, exception, or fault. Interrupt
> handler are executing between instructions and traps/exceptions/faults
> will either execute instead of the faulty instruction or after is has
> been executed. In all these cases, each instruction can be seen as
> executing atomically wrt the local CPU. This is exactly what permits
> asm-i386/local.h to define out the LOCK prefix for UP kernels.

I think what I'm looking for but don't know if it's true:
    The cmpxchg (for example) at the kernel process context will not
    clobber or be clobbered by a cmpxchg done to the same local_t
    performed at the kernel interrupt context by the same CPU.

If that's not true, then it would be good to add that as another
restriction to usage.

> I use the same trick UP kernel are using, but I deploy it in SMP
> context, but I require the CPU to be the only one to access the memory
> locations written to by the local ops.
> 
> Basically, since the memory location is _not_ shared across CPUs for
> writing, we can safely write to it without holding the LOCK signal.

ok.

...
> > Note: I already missed the one critical sentence about only the "owning"
> > CPU can write the value....there seem to be other limitations as well
> > with respect to interrupts.
> > 
> 
> Ok, let's give a try at a clear statement:
> 
> - Variables touched by local ops must be per cpu variables.
> - _Only_ the CPU owner of these variables must write to them.
> - This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
>   to update its local_t variables.
> - Preemption (or interrupts) must be disabled when using local ops in
>   process context to   make sure the process won't be migrated to a
>   different CPU between getting the per-cpu variable and doing the
>   actual local op.
> - When using local ops in interrupt context, no special care must be
>   taken on a mainline kernel, since they will run on the local CPU with
>   preemption already disabled. I suggest, however, to explicitly
>   disable preemption anyway to make sure it will still work correctly on
>   -rt kernels.
> - Reading the local cpu variable will provide the current copy of the
>   variable.
> - Reads of these variables can be done from any CPU, because updates to
>   "long", aligned, variables are always atomic. Since no memory
>   synchronization is done by the writer CPU, an outdated copy of the
>   variable can be read when reading some _other_ cpu's variables.

Perfect! Ship it! ;)
Can you submit a patch to add the above to Documentation/local_ops.txt ?

...
> > Looks fine to me. Add your "Signed-off-by" and submit to DaveM
> > since he seems to be the maintainer of atomic_ops.txt.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

heh...I think DaveM will want it in git or universal patch form. :)
Patch appended below.

thanks!
grant

Add document reference and simple use example of local_t.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

--- linux-2.6.22.5-ORIG/Documentation/atomic_ops.txt	2007-08-22 16:23:54.000000000 -0700
+++ linux-2.6.22.5-ggg/Documentation/atomic_ops.txt	2007-08-28 22:57:26.000000000 -0700
@@ -7,6 +7,11 @@
 maintainers on how to implement atomic counter, bitops, and spinlock
 interfaces properly.
 
+	atomic_t should be used to provide a type with update primitives
+executed atomically from any CPU.  If the counter is per CPU and only
+updated by one CPU, local_t is probably more appropriate.  Please see
+Documentation/local_ops.txt for the semantics of local_t.
+
 	The atomic_t type should be defined as a signed integer.
 Also, it should be made opaque such that any kind of cast to a normal
 C integer type will fail.  Something like the following should
-
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