Heiko Carstens wrote:
On Wed, Aug 08, 2007 at 02:31:15PM -0700, Andrew Morton wrote:
On Wed, 08 Aug 2007 17:08:44 -0400
Chris Snook <[email protected]> wrote:
Heiko Carstens wrote:
On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote:
From: Heiko Carstens <[email protected]>
Date: Wed, 8 Aug 2007 11:33:00 +0200
Just saw this while grepping for atomic_reads in a while loops.
Maybe we should re-add the volatile to atomic_t. Not sure.
I think whatever the choice, it should be done consistently
on every architecture.
It's just asking for trouble if your arch does it differently from
every other.
Well..currently it's i386/x86_64 and s390 which have no volatile
in atomic_t. And yes, of course I agree it should be consistent
across all architectures. But it isn't.
Based on recent discussion, it's pretty clear that there's a lot of
confusion about this. A lot of people (myself included, until I thought
about it long and hard) will reasonably assume that calling
atomic_read() will actually read the value from memory. Leaving out the
volatile declaration seems like a pessimization to me. If you force
people to use barrier() everywhere they're working with atomic_t, it
will force re-reads of all the non-atomic data in use as well, which
will cause more memory fetches of things that generally don't need
barrier(). That and it's a bug waiting to happen.
Andi -- your thoughts on the matter?
I'm not Andi, but this not-Andi thinks that permitting the compiler to cache
the results of atomic_read() is dumb.
Ok, how about this:
Subject: [PATCH] Add 'volatile' to atomic_t again.
From: Heiko Carstens <[email protected]>
This basically reverts f9e9dcb38f5106fa8cdac04a9e967d5487f1cd20 which
removed 'volatile' from atomic_t for i386/x86_64. Reason for this
is to make sure that code like
while (atomic_read(&whatever));
continues to work.
Otherwise the compiler might generate code that will loop forever.
Also this makes sure atomic_t is the same across all architectures.
Cc: Andi Kleen <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
---
s390 patch will go in via Martin if this is accepted.
include/asm-i386/atomic.h | 2 +-
include/asm-x86_64/atomic.h | 2 +-
3 files changed, 4 insertions(+), 4 deletions(-)
Index: linux-2.6/include/asm-i386/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-i386/atomic.h
+++ linux-2.6/include/asm-i386/atomic.h
@@ -15,7 +15,7 @@
* on us. We need to use _exactly_ the address the user gave us,
* not some alias that contains the same information.
*/
-typedef struct { int counter; } atomic_t;
+typedef struct { volatile int counter; } atomic_t;
#define ATOMIC_INIT(i) { (i) }
Index: linux-2.6/include/asm-x86_64/atomic.h
===================================================================
--- linux-2.6.orig/include/asm-x86_64/atomic.h
+++ linux-2.6/include/asm-x86_64/atomic.h
@@ -22,7 +22,7 @@
* on us. We need to use _exactly_ the address the user gave us,
* not some alias that contains the same information.
*/
-typedef struct { int counter; } atomic_t;
+typedef struct { volatile int counter; } atomic_t;
#define ATOMIC_INIT(i) { (i) }
Good so far, but we need to fix it on non-SMP architectures too, since
drivers may use atomic_t in interrupt code. Ideally I'd like to be able
to remove a whole bunch of barriers, since they cause a lot of needless
re-fetches for everything else in the loop. We should also document the
semantics of atomic_t to ensure consistency in the future.
-- Chris
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