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 <[email protected]>
--- 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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