Messages in this thread | | | Date | Wed, 08 Aug 2007 18:38:25 -0400 | From | Chris Snook <> | Subject | Re: [patch] ipvs: force read of atomic_t in while loop |
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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 <csnook@redhat.com> wrote: >> >>> Heiko Carstens wrote: >>>> On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote: >>>>> From: Heiko Carstens <heiko.carstens@de.ibm.com> >>>>> 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 <heiko.carstens@de.ibm.com> > > 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 <ak@suse.de> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> > Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com> > --- > > 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.
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