linux-kernel - Re: [PATCH] local_t : Documentation

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Date:	Tue, 9 Jan 2007 18:21:55 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Pavel Machek <pavel@....cz>
Cc:	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] local_t : Documentation - update

* Pavel Machek (pavel@....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 :
-- 
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