lkml.org 
[lkml]   [2007]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH] local_t : Documentation - update
* Pavel Machek (pavel@ucw.cz) wrote:
> Hi!
>
> AFAICT this fails to mention... Is local_t as big as int? As big as
> long? Or perhaps smaller because high bits may be needed for locking?
>
> Pavel
>

Hi Pavel,

Here is an update that adds the information you mentionned in this reply and the
one to Andrew. Thanks for the comments.

Mathieu


index dfeec94..bd854b3 100644
--- a/Documentation/local_ops.txt
+++ b/Documentation/local_ops.txt
@@ -22,6 +22,13 @@ require disabling interrupts to protect from interrupt handlers and it permits
coherent counters in NMI handlers. It is especially useful for tracing purposes
and for various performance monitoring counters.

+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. It is however
+permitted to read local_t data from any CPU : it will then appear to be written
+out of order wrt other memory writes on the owner CPU.
+

* Implementation for a given architecture

@@ -31,6 +38,12 @@ i386 and x86_64) and any SMP sychronization barrier. If the architecture does
not have a different behavior between SMP and UP, including asm-generic/local.h
in your archtecture's local.h is sufficient.

+The local_t type is defined as an opaque signed long by embedding an
+atomic_long_t inside a structure. This is made so a cast from this type to a
+long fails. The definition looks like :
+
+typedef struct { atomic_long_t a; } local_t;
+

* How to use local atomic operations

@@ -42,6 +55,8 @@ static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);

* Counting

+Counting is done on all the bits of a signed long.
+
In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
operations : it makes sure that preemption is disabled around write access to
the per cpu variable. For instance :
--
OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg
Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
\
 
 \ /
  Last update: 2007-01-10 01:07    [W:0.221 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site