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

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

On Tue, Aug 28, 2007 at 07:50:18AM -0400, Mathieu Desnoyers wrote:
...
> > A few questions/nits:
> > o Did you attempt quantify how many places in the kernel could use this?
> >   I'm just trying to get a feel for how useful this really is vs just 
> >   using existing mechanisms (that people understand) to implement a 
> >   non-SMP-safe counter that protects updates (writes) against interrupts.
> >   If you did, adding some referencs to local_ops.txt would be helpful
> >   so folks could look for examples of "correct usage".
> > 
> 
> Good question. Since it is useful to implement fast, interrupt
> reentrant, counters of any kind without disabling interrupts, I think it
> could be vastely used in the kernel. I also use it in my LTTng kernel
> tracer implementation to provide very fast buffer management. It is used
> in LTTng, but could be used for most kind of buffering management too;
> meaning that we could manage buffers without disabling interrupts.
> 
> 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?

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.


> > o How can a local_t counter protect updates (writes) against interrupts 
> >   but not preemption?
> >   I always thought preemption required some sort of interrupt or trap.
> >   Maybe the local_ops.txt explains that and I just missed it.
> > 
> 
> "Local atomic operations only guarantee variable modification atomicity
> wrt the CPU which owns the data. Therefore, care must taken to make sure
> that only one CPU writes to the local_t data. This is done by using per
> cpu data and making sure that we modify it from within a preemption safe
> context." -> therefore, preemption must be disabled around local ops
> usage. This is required to be pinned to one CPU anyway.

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.

> >   DaveM explained updates "in flight" would not be visible to interrupts
> >   and I suspect that's the answer to my question....but then I don't "feel
> >   good" the local_ops are safe to update in interrupts _and_ the process
> >   context kernel.  Maybe the relationship between local_ops, preemption,
> >   and interrupts could be explained more carefully in local_ops.txt.
> > 
> 
> Does the paragraph above explain it enough or should I add some more
> explanation ?

Please add a bit more detail. If DaveM is correct (he normally is), then
there must be limits on how the local_t can be used in the kernel process
and interrupt contexts. I'd like those rules spelled out very clearly
since it's easy to get wrong and tracking down such a bug is quite painful.

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.

> > o OK to add a reference for local_ops.txt to atomic_ops.txt?
> >   They are obviously related and anyone "discovering" one of the docs
> >   should be made aware of the other.
> >   Patch+log entry appended below. Please sign-off if that's ok with you.
> > 
> 
> I'm perfectly ok with the idea, but suggest a small modification. See
> below.

Looks fine to me. Add your "Signed-off-by" and submit to DaveM
since he seems to be the maintainer of atomic_ops.txt.

cheers,
grant

> 
> > 
> > thanks,
> > grant
> > 
> > Diff+Commit entry against 2.6.22.5:
> > 
> > local_t is a variant of atomic_t and has related ops to match.
> > Add reference for local_t documentation to atomic_ops.txt. 
> > 
> > Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
> > 
> > 
> > --- 2.6.22.5-ORIG/Documentation/atomic_ops.txt	2007-08-27 22:50:27.000000000 -0700
> > +++ 2.6.22.5-ggg/Documentation/atomic_ops.txt	2007-08-27 22:54:44.000000000 -0700
> > @@ -14,6 +14,10 @@
> >  
> >  	typedef struct { volatile int counter; } atomic_t;
> >  
> > +local_t is very similar to atomic_t. 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 first operations to implement for atomic_t's are the
> >  initializers and plain reads.
> >  
> 
> The text snippet is good, but I am not sure it belongs between the
> description of atomic_t type and its initializers. What if we do
> something like: (with context, I tried to explain the distinction
> between atomic_t and local_t some more)
> 
> 
>                 Semantics and Behavior of Atomic and
>                          Bitmask Operations
> 
>                           David S. Miller
> 
>         This document is intended to serve as a guide to Linux port
> 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
> suffice: 
> 
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68