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SubjectRe: 64-bit ppc rwsem
From
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Wed, 18 Aug 2010 15:03:23 +1000

> I tried various tricks but so far they didn't work. I'll have another
> look tomorrow, but I may end up having to keep all the crap typecasts.

The casts are pretty much unavoidable.

Here's what I'm going to end up using on sparc64:

--------------------
sparc64: Make rwsems 64-bit.
Basically tip-off the powerpc code, use a 64-bit type and atomic64_t
interfaces for the implementation.

This gets us off of the by-hand asm code I wrote, which frankly I
think probably ruins I-cache hit rates.

The idea was the keep the call chains less deep, but anything taking
the rw-semaphores probably is also calling other stuff and therefore
already has allocated a stack-frame. So no real stack frame savings
ever.

Ben H. has posted patches to make powerpc use 64-bit too and with some
abstractions we can probably use a shared header file somewhere.

Signed-off-by: David S. Miller <davem@davemloft.net>
---
arch/sparc/include/asm/rwsem-const.h | 12 ---
arch/sparc/include/asm/rwsem.h | 120 +++++++++++++++++++++----
arch/sparc/lib/Makefile | 2 +-
arch/sparc/lib/rwsem_64.S | 163 ----------------------------------
4 files changed, 104 insertions(+), 193 deletions(-)
delete mode 100644 arch/sparc/include/asm/rwsem-const.h
delete mode 100644 arch/sparc/lib/rwsem_64.S
diff --git a/arch/sparc/include/asm/rwsem-const.h b/arch/sparc/include/asm/rwsem-const.h
deleted file mode 100644
index e4c61a1..0000000
--- a/arch/sparc/include/asm/rwsem-const.h
+++ /dev/null
@@ -1,12 +0,0 @@
-/* rwsem-const.h: RW semaphore counter constants. */
-#ifndef _SPARC64_RWSEM_CONST_H
-#define _SPARC64_RWSEM_CONST_H
-
-#define RWSEM_UNLOCKED_VALUE 0x00000000
-#define RWSEM_ACTIVE_BIAS 0x00000001
-#define RWSEM_ACTIVE_MASK 0x0000ffff
-#define RWSEM_WAITING_BIAS (-0x00010000)
-#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
-#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
-
-#endif /* _SPARC64_RWSEM_CONST_H */
diff --git a/arch/sparc/include/asm/rwsem.h b/arch/sparc/include/asm/rwsem.h
index 6e56210..a2b4302 100644
--- a/arch/sparc/include/asm/rwsem.h
+++ b/arch/sparc/include/asm/rwsem.h
@@ -15,16 +15,21 @@

#include <linux/list.h>
#include <linux/spinlock.h>
-#include <asm/rwsem-const.h>

struct rwsem_waiter;

struct rw_semaphore {
- signed int count;
- spinlock_t wait_lock;
- struct list_head wait_list;
+ signed long count;
+#define RWSEM_UNLOCKED_VALUE 0x00000000L
+#define RWSEM_ACTIVE_BIAS 0x00000001L
+#define RWSEM_ACTIVE_MASK 0xffffffffL
+#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
+#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
+#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+ spinlock_t wait_lock;
+ struct list_head wait_list;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
- struct lockdep_map dep_map;
+ struct lockdep_map dep_map;
#endif
};

@@ -41,6 +46,11 @@ struct rw_semaphore {
#define DECLARE_RWSEM(name) \
struct rw_semaphore name = __RWSEM_INITIALIZER(name)

+extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
+
extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
struct lock_class_key *key);

@@ -51,27 +61,103 @@ do { \
__init_rwsem((sem), #sem, &__key); \
} while (0)

-extern void __down_read(struct rw_semaphore *sem);
-extern int __down_read_trylock(struct rw_semaphore *sem);
-extern void __down_write(struct rw_semaphore *sem);
-extern int __down_write_trylock(struct rw_semaphore *sem);
-extern void __up_read(struct rw_semaphore *sem);
-extern void __up_write(struct rw_semaphore *sem);
-extern void __downgrade_write(struct rw_semaphore *sem);
+/*
+ * lock for reading
+ */
+static inline void __down_read(struct rw_semaphore *sem)
+{
+ if (unlikely(atomic64_inc_return((atomic64_t *)(&sem->count)) <= 0L))
+ rwsem_down_read_failed(sem);
+}
+
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+ long tmp;
+
+ while ((tmp = sem->count) >= 0L) {
+ if (tmp == cmpxchg(&sem->count, tmp,
+ tmp + RWSEM_ACTIVE_READ_BIAS)) {
+ return 1;
+ }
+ }
+ return 0;
+}

+/*
+ * lock for writing
+ */
static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
{
- __down_write(sem);
+ long tmp;
+
+ tmp = atomic64_add_return(RWSEM_ACTIVE_WRITE_BIAS,
+ (atomic64_t *)(&sem->count));
+ if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+ rwsem_down_write_failed(sem);
}

-static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
+static inline void __down_write(struct rw_semaphore *sem)
{
- return atomic_add_return(delta, (atomic_t *)(&sem->count));
+ __down_write_nested(sem, 0);
+}
+
+static inline int __down_write_trylock(struct rw_semaphore *sem)
+{
+ long tmp;
+
+ tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+ RWSEM_ACTIVE_WRITE_BIAS);
+ return tmp == RWSEM_UNLOCKED_VALUE;
}

-static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
+/*
+ * unlock after reading
+ */
+static inline void __up_read(struct rw_semaphore *sem)
+{
+ long tmp;
+
+ tmp = atomic64_dec_return((atomic64_t *)(&sem->count));
+ if (unlikely(tmp < -1L && (tmp & RWSEM_ACTIVE_MASK) == 0L))
+ rwsem_wake(sem);
+}
+
+/*
+ * unlock after writing
+ */
+static inline void __up_write(struct rw_semaphore *sem)
+{
+ if (unlikely(atomic64_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
+ (atomic64_t *)(&sem->count)) < 0L))
+ rwsem_wake(sem);
+}
+
+/*
+ * implement atomic add functionality
+ */
+static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
+{
+ atomic64_add(delta, (atomic64_t *)(&sem->count));
+}
+
+/*
+ * downgrade write lock to read lock
+ */
+static inline void __downgrade_write(struct rw_semaphore *sem)
+{
+ long tmp;
+
+ tmp = atomic64_add_return(-RWSEM_WAITING_BIAS, (atomic64_t *)(&sem->count));
+ if (tmp < 0L)
+ rwsem_downgrade_wake(sem);
+}
+
+/*
+ * implement exchange and add functionality
+ */
+static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
{
- atomic_add(delta, (atomic_t *)(&sem->count));
+ return atomic64_add_return(delta, (atomic64_t *)(&sem->count));
}

static inline int rwsem_is_locked(struct rw_semaphore *sem)
diff --git a/arch/sparc/lib/Makefile b/arch/sparc/lib/Makefile
index c4b5e03..fa4c3ea 100644
--- a/arch/sparc/lib/Makefile
+++ b/arch/sparc/lib/Makefile
@@ -15,7 +15,7 @@ lib-$(CONFIG_SPARC32) += divdi3.o udivdi3.o
lib-$(CONFIG_SPARC32) += copy_user.o locks.o
lib-y += atomic_$(BITS).o
lib-$(CONFIG_SPARC32) += lshrdi3.o ashldi3.o
-lib-y += rwsem_$(BITS).o
+lib-$(CONFIG_SPARC32) += rwsem_$(BITS).o
lib-$(CONFIG_SPARC32) += muldi3.o bitext.o cmpdi2.o

lib-$(CONFIG_SPARC64) += copy_page.o clear_page.o bzero.o
diff --git a/arch/sparc/lib/rwsem_64.S b/arch/sparc/lib/rwsem_64.S
deleted file mode 100644
index 91a7d29..0000000
--- a/arch/sparc/lib/rwsem_64.S
+++ /dev/null
@@ -1,163 +0,0 @@
-/* rwsem.S: RW semaphore assembler.
- *
- * Written by David S. Miller (davem@redhat.com), 2001.
- * Derived from asm-i386/rwsem.h
- */
-
-#include <asm/rwsem-const.h>
-
- .section .sched.text, "ax"
-
- .globl __down_read
-__down_read:
-1: lduw [%o0], %g1
- add %g1, 1, %g7
- cas [%o0], %g1, %g7
- cmp %g1, %g7
- bne,pn %icc, 1b
- add %g7, 1, %g7
- cmp %g7, 0
- bl,pn %icc, 3f
- nop
-2:
- retl
- nop
-3:
- save %sp, -192, %sp
- call rwsem_down_read_failed
- mov %i0, %o0
- ret
- restore
- .size __down_read, .-__down_read
-
- .globl __down_read_trylock
-__down_read_trylock:
-1: lduw [%o0], %g1
- add %g1, 1, %g7
- cmp %g7, 0
- bl,pn %icc, 2f
- mov 0, %o1
- cas [%o0], %g1, %g7
- cmp %g1, %g7
- bne,pn %icc, 1b
- mov 1, %o1
-2: retl
- mov %o1, %o0
- .size __down_read_trylock, .-__down_read_trylock
-
- .globl __down_write
-__down_write:
- sethi %hi(RWSEM_ACTIVE_WRITE_BIAS), %g1
- or %g1, %lo(RWSEM_ACTIVE_WRITE_BIAS), %g1
-1:
- lduw [%o0], %g3
- add %g3, %g1, %g7
- cas [%o0], %g3, %g7
- cmp %g3, %g7
- bne,pn %icc, 1b
- cmp %g7, 0
- bne,pn %icc, 3f
- nop
-2: retl
- nop
-3:
- save %sp, -192, %sp
- call rwsem_down_write_failed
- mov %i0, %o0
- ret
- restore
- .size __down_write, .-__down_write
-
- .globl __down_write_trylock
-__down_write_trylock:
- sethi %hi(RWSEM_ACTIVE_WRITE_BIAS), %g1
- or %g1, %lo(RWSEM_ACTIVE_WRITE_BIAS), %g1
-1:
- lduw [%o0], %g3
- cmp %g3, 0
- bne,pn %icc, 2f
- mov 0, %o1
- add %g3, %g1, %g7
- cas [%o0], %g3, %g7
- cmp %g3, %g7
- bne,pn %icc, 1b
- mov 1, %o1
-2: retl
- mov %o1, %o0
- .size __down_write_trylock, .-__down_write_trylock
-
- .globl __up_read
-__up_read:
-1:
- lduw [%o0], %g1
- sub %g1, 1, %g7
- cas [%o0], %g1, %g7
- cmp %g1, %g7
- bne,pn %icc, 1b
- cmp %g7, 0
- bl,pn %icc, 3f
- nop
-2: retl
- nop
-3: sethi %hi(RWSEM_ACTIVE_MASK), %g1
- sub %g7, 1, %g7
- or %g1, %lo(RWSEM_ACTIVE_MASK), %g1
- andcc %g7, %g1, %g0
- bne,pn %icc, 2b
- nop
- save %sp, -192, %sp
- call rwsem_wake
- mov %i0, %o0
- ret
- restore
- .size __up_read, .-__up_read
-
- .globl __up_write
-__up_write:
- sethi %hi(RWSEM_ACTIVE_WRITE_BIAS), %g1
- or %g1, %lo(RWSEM_ACTIVE_WRITE_BIAS), %g1
-1:
- lduw [%o0], %g3
- sub %g3, %g1, %g7
- cas [%o0], %g3, %g7
- cmp %g3, %g7
- bne,pn %icc, 1b
- sub %g7, %g1, %g7
- cmp %g7, 0
- bl,pn %icc, 3f
- nop
-2:
- retl
- nop
-3:
- save %sp, -192, %sp
- call rwsem_wake
- mov %i0, %o0
- ret
- restore
- .size __up_write, .-__up_write
-
- .globl __downgrade_write
-__downgrade_write:
- sethi %hi(RWSEM_WAITING_BIAS), %g1
- or %g1, %lo(RWSEM_WAITING_BIAS), %g1
-1:
- lduw [%o0], %g3
- sub %g3, %g1, %g7
- cas [%o0], %g3, %g7
- cmp %g3, %g7
- bne,pn %icc, 1b
- sub %g7, %g1, %g7
- cmp %g7, 0
- bl,pn %icc, 3f
- nop
-2:
- retl
- nop
-3:
- save %sp, -192, %sp
- call rwsem_downgrade_wake
- mov %i0, %o0
- ret
- restore
- .size __downgrade_write, .-__downgrade_write
--
1.7.2.1


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