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

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

[Mathieu Desnoyers]

Use the new generic cmpxchg_local (disables interrupt). Also use the generic
cmpxchg as fallback if SMP is not set.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: clameter@sgi.com
CC: parisc-linux@parisc-linux.org
---
 include/asm-parisc/atomic.h |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

Index: linux-2.6-lttng/include/asm-parisc/atomic.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-parisc/atomic.h	2007-07-20 19:44:40.000000000 -0400
+++ linux-2.6-lttng/include/asm-parisc/atomic.h	2007-07-20 19:44:47.000000000 -0400
@@ -122,6 +122,35 @@ __cmpxchg(volatile void *ptr, unsigned l
 				    (unsigned long)_n_, sizeof(*(ptr))); \
   })
 
+#include <asm-generic/cmpxchg-local.h>
+
+static inline unsigned long __cmpxchg_local(volatile void *ptr,
+				      unsigned long old,
+				      unsigned long new_, int size)
+{
+	switch (size) {
+#ifdef CONFIG_64BIT
+	case 8:	return __cmpxchg_u64((unsigned long *)ptr, old, new_);
+#endif
+	case 4:	return __cmpxchg_u32(ptr, old, new_);
+	default:
+		return __cmpxchg_local_generic(ptr, old, new_, size);
+	}
+}
+
+/*
+ * cmpxchg_local and cmpxchg64_local are atomic wrt current CPU. Always make
+ * them available.
+ */
+#define cmpxchg_local(ptr,o,n)					  	\
+     (__typeof__(*(ptr)))__cmpxchg_local((ptr), (unsigned long)(o),	\
+			   	 (unsigned long)(n), sizeof(*(ptr)))
+#ifdef CONFIG_64BIT
+#define cmpxchg64_local(ptr,o,n) cmpxchg_local((ptr), (o), (n))
+#else
+#define cmpxchg64_local(ptr,o,n) __cmpxchg64_local_generic((ptr), (o), (n))
+#endif
+
 /* Note that we need not lock read accesses - aligned word writes/reads
  * are atomic, so a reader never sees unconsistent values.
  *

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc

[Grant Grundler]

On Sun, Aug 12, 2007 at 10:54:49AM -0400, Mathieu Desnoyers wrote:
> Use the new generic cmpxchg_local (disables interrupt). Also use the generic
> cmpxchg as fallback if SMP is not set.

Mathieu,
thanks for adding __cmpxchg_local to parisc.... but why do we need it?

By definition, atomic operators are, well, atomic.

I searched for __cmpxchg_local and found this reference:
    http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1337.html

but the "root" of that thread (Dec 20, 2006):
    http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1334.html

Doesn't explain the difference between "local" and "non-local" either.
Per CPU data should only need memory barriers (in some cases) and
protection against interrupts (in probably more cases). So I'm not
understanding why a new set of APIs is needed.

Can you add a description to Documentation/atomic_ops.txt ?
*sigh* sorry for being "late to the party" on this one...

cheers,
grant

> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> CC: clameter@sgi.com
> CC: parisc-linux@parisc-linux.org
> ---
>  include/asm-parisc/atomic.h |   29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> Index: linux-2.6-lttng/include/asm-parisc/atomic.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/asm-parisc/atomic.h	2007-07-20 19:44:40.000000000 -0400
> +++ linux-2.6-lttng/include/asm-parisc/atomic.h	2007-07-20 19:44:47.000000000 -0400
> @@ -122,6 +122,35 @@ __cmpxchg(volatile void *ptr, unsigned l
>  				    (unsigned long)_n_, sizeof(*(ptr))); \
>    })
>  
> +#include <asm-generic/cmpxchg-local.h>
> +
> +static inline unsigned long __cmpxchg_local(volatile void *ptr,
> +				      unsigned long old,
> +				      unsigned long new_, int size)
> +{
> +	switch (size) {
> +#ifdef CONFIG_64BIT
> +	case 8:	return __cmpxchg_u64((unsigned long *)ptr, old, new_);
> +#endif
> +	case 4:	return __cmpxchg_u32(ptr, old, new_);
> +	default:
> +		return __cmpxchg_local_generic(ptr, old, new_, size);
> +	}
> +}
> +
> +/*
> + * cmpxchg_local and cmpxchg64_local are atomic wrt current CPU. Always make
> + * them available.
> + */
> +#define cmpxchg_local(ptr,o,n)					  	\
> +     (__typeof__(*(ptr)))__cmpxchg_local((ptr), (unsigned long)(o),	\
> +			   	 (unsigned long)(n), sizeof(*(ptr)))
> +#ifdef CONFIG_64BIT
> +#define cmpxchg64_local(ptr,o,n) cmpxchg_local((ptr), (o), (n))
> +#else
> +#define cmpxchg64_local(ptr,o,n) __cmpxchg64_local_generic((ptr), (o), (n))
> +#endif
> +
>  /* Note that we need not lock read accesses - aligned word writes/reads
>   * are atomic, so a reader never sees unconsistent values.
>   *
> 
> -- 
> Mathieu Desnoyers
> Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> _______________________________________________
> parisc-linux mailing list
> parisc-linux@lists.parisc-linux.org
> http://lists.parisc-linux.org/mailman/listinfo/parisc-linux
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Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc

[Mathieu Desnoyers]

* Grant Grundler (grundler@parisc-linux.org) wrote:
> On Sun, Aug 12, 2007 at 10:54:49AM -0400, Mathieu Desnoyers wrote:
> > Use the new generic cmpxchg_local (disables interrupt). Also use the generic
> > cmpxchg as fallback if SMP is not set.
> 
> Mathieu,
> thanks for adding __cmpxchg_local to parisc.... but why do we need it?
> 
> By definition, atomic operators are, well, atomic.
> 
> I searched for __cmpxchg_local and found this reference:
>     http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1337.html
> 
> but the "root" of that thread (Dec 20, 2006):
>     http://www.ussg.iu.edu/hypermail/linux/kernel/0612.2/1334.html
> 
> Doesn't explain the difference between "local" and "non-local" either.
> Per CPU data should only need memory barriers (in some cases) and
> protection against interrupts (in probably more cases). So I'm not
> understanding why a new set of APIs is needed.
> 
> Can you add a description to Documentation/atomic_ops.txt ?
> *sigh* sorry for being "late to the party" on this one...
> 

Does Documentation/local_ops.txt answer your questions ? If not, please
tell me and I'll gladly explain more.

Mathieu

> cheers,
> grant
> 
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > CC: clameter@sgi.com
> > CC: parisc-linux@parisc-linux.org
> > ---
> >  include/asm-parisc/atomic.h |   29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> > 
> > Index: linux-2.6-lttng/include/asm-parisc/atomic.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/asm-parisc/atomic.h	2007-07-20 19:44:40.000000000 -0400
> > +++ linux-2.6-lttng/include/asm-parisc/atomic.h	2007-07-20 19:44:47.000000000 -0400
> > @@ -122,6 +122,35 @@ __cmpxchg(volatile void *ptr, unsigned l
> >  				    (unsigned long)_n_, sizeof(*(ptr))); \
> >    })
> >  
> > +#include <asm-generic/cmpxchg-local.h>
> > +
> > +static inline unsigned long __cmpxchg_local(volatile void *ptr,
> > +				      unsigned long old,
> > +				      unsigned long new_, int size)
> > +{
> > +	switch (size) {
> > +#ifdef CONFIG_64BIT
> > +	case 8:	return __cmpxchg_u64((unsigned long *)ptr, old, new_);
> > +#endif
> > +	case 4:	return __cmpxchg_u32(ptr, old, new_);
> > +	default:
> > +		return __cmpxchg_local_generic(ptr, old, new_, size);
> > +	}
> > +}
> > +
> > +/*
> > + * cmpxchg_local and cmpxchg64_local are atomic wrt current CPU. Always make
> > + * them available.
> > + */
> > +#define cmpxchg_local(ptr,o,n)					  	\
> > +     (__typeof__(*(ptr)))__cmpxchg_local((ptr), (unsigned long)(o),	\
> > +			   	 (unsigned long)(n), sizeof(*(ptr)))
> > +#ifdef CONFIG_64BIT
> > +#define cmpxchg64_local(ptr,o,n) cmpxchg_local((ptr), (o), (n))
> > +#else
> > +#define cmpxchg64_local(ptr,o,n) __cmpxchg64_local_generic((ptr), (o), (n))
> > +#endif
> > +
> >  /* Note that we need not lock read accesses - aligned word writes/reads
> >   * are atomic, so a reader never sees unconsistent values.
> >   *
> > 
> > -- 
> > Mathieu Desnoyers
> > Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> > _______________________________________________
> > parisc-linux mailing list
> > parisc-linux@lists.parisc-linux.org
> > http://lists.parisc-linux.org/mailman/listinfo/parisc-linux

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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Re: [parisc-linux] [patch 15/23] Add cmpxchg_local to parisc

[Grant Grundler]

On Mon, Aug 27, 2007 at 05:11:40PM -0400, Mathieu Desnoyers wrote:
...
> > Can you add a description to Documentation/atomic_ops.txt ?
> > *sigh* sorry for being "late to the party" on this one...
> 
> Does Documentation/local_ops.txt answer your questions ? If not, please
> tell me and I'll gladly explain more.

Yes, it does mostly - thanks. 

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".

o Wording in local_ops.txt: "on the 
	"... it will then appear to be written out of order wrt
	 other memory writes on the owner CPU."

  I'd like to suggest "by the owner CPU".

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.

  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.

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.


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

[Mathieu Desnoyers]

* Grant Grundler (grundler@parisc-linux.org) wrote:
> On Mon, Aug 27, 2007 at 05:11:40PM -0400, Mathieu Desnoyers wrote:
> ...
> > > Can you add a description to Documentation/atomic_ops.txt ?
> > > *sigh* sorry for being "late to the party" on this one...
> > 
> > Does Documentation/local_ops.txt answer your questions ? If not, please
> > tell me and I'll gladly explain more.
> 
> Yes, it does mostly - thanks. 
> 
> 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...

> o Wording in local_ops.txt: "on the 
> 	"... it will then appear to be written out of order wrt
> 	 other memory writes on the owner CPU."
> 
>   I'd like to suggest "by the owner CPU".
> 

Ok, fixing.. thanks!

> 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.

>   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 ?

> 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.

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

[Grant Grundler]

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

[Mathieu Desnoyers]

* Grant Grundler (grundler@parisc-linux.org) wrote:
> 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?
> 

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 ;)

> 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!) ;)

> 
> > > 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.
> 

Quoting Intel 64 and IA-32 Architectures Software Developer's Manual

3.2 Instructions
LOCK - Assert LOCK# Signal Prefix

Causes the processor's LOCK# signal to be asserted during execution of
the accompanying instruction (turns the instruction into an atomic
instruction). In a multiprocessor environment, the LOCK# signal insures
that the processor has exclusive use of any shared memory while the
signal is asserted.

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 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.


> > >   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.
> 

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.


> > > 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.
> 

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

Thanks,

Mathieu

> 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

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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