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authorAnton Blanchard <anton@samba.org>2010-05-17 14:34:57 +1000
committerLinus Torvalds <torvalds@linux-foundation.org>2010-05-17 07:57:27 -0700
commit81880d603d00c645e0890d0a44d50711c503b72b (patch)
tree69d4bb390d8d2595017192b51feb08e2fed3c26a
parentf3d46f9d3194e0329216002a8724d4c0957abc79 (diff)
downloadlinux-2.6.34-ux500-81880d603d00c645e0890d0a44d50711c503b72b.tar.gz
atomic_t: Remove volatile from atomic_t definition
When looking at a performance problem on PowerPC, I noticed some awful code generation: c00000000051fc98: 3b 60 00 01 li r27,1 ... c00000000051fca0: 3b 80 00 00 li r28,0 ... c00000000051fcdc: 93 61 00 70 stw r27,112(r1) c00000000051fce0: 93 81 00 74 stw r28,116(r1) c00000000051fce4: 81 21 00 70 lwz r9,112(r1) c00000000051fce8: 80 01 00 74 lwz r0,116(r1) c00000000051fcec: 7d 29 07 b4 extsw r9,r9 c00000000051fcf0: 7c 00 07 b4 extsw r0,r0 c00000000051fcf4: 7c 20 04 ac lwsync c00000000051fcf8: 7d 60 f8 28 lwarx r11,0,r31 c00000000051fcfc: 7c 0b 48 00 cmpw r11,r9 c00000000051fd00: 40 c2 00 10 bne- c00000000051fd10 c00000000051fd04: 7c 00 f9 2d stwcx. r0,0,r31 c00000000051fd08: 40 c2 ff f0 bne+ c00000000051fcf8 c00000000051fd0c: 4c 00 01 2c isync We create two constants, write them out to the stack, read them straight back in and sign extend them. What a mess. It turns out this bad code is a result of us defining atomic_t as a volatile int. We removed the volatile attribute from the powerpc atomic_t definition years ago, but commit ea435467500612636f8f4fb639ff6e76b2496e4b (atomic_t: unify all arch definitions) added it back in. To dig up an old quote from Linus: > The fact is, volatile on data structures is a bug. It's a wart in the C > language. It shouldn't be used. > > Volatile accesses in *code* can be ok, and if we have "atomic_read()" > expand to a "*(volatile int *)&(x)->value", then I'd be ok with that. > > But marking data structures volatile just makes the compiler screw up > totally, and makes code for initialization sequences etc much worse. And screw up it does :) With the volatile removed, we see much more reasonable code generation: c00000000051f5b8: 3b 60 00 01 li r27,1 ... c00000000051f5c0: 3b 80 00 00 li r28,0 ... c00000000051fc7c: 7c 20 04 ac lwsync c00000000051fc80: 7c 00 f8 28 lwarx r0,0,r31 c00000000051fc84: 7c 00 d8 00 cmpw r0,r27 c00000000051fc88: 40 c2 00 10 bne- c00000000051fc98 c00000000051fc8c: 7f 80 f9 2d stwcx. r28,0,r31 c00000000051fc90: 40 c2 ff f0 bne+ c00000000051fc80 c00000000051fc94: 4c 00 01 2c isync Six instructions less. Signed-off-by: Anton Blanchard <anton@samba.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/types.h4
1 files changed, 2 insertions, 2 deletions
diff --git a/include/linux/types.h b/include/linux/types.h
index c42724f8c80..23d237a075e 100644
--- a/include/linux/types.h
+++ b/include/linux/types.h
@@ -188,12 +188,12 @@ typedef u32 phys_addr_t;
typedef phys_addr_t resource_size_t;
typedef struct {
- volatile int counter;
+ int counter;
} atomic_t;
#ifdef CONFIG_64BIT
typedef struct {
- volatile long counter;
+ long counter;
} atomic64_t;
#endif