Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

Peter Zijlstra
2019-02-11T11:58:55Z

[-- Attachment #1: Type: text/plain, Size: 3269 bytes --]

On Sun, Feb 10, 2019 at 09:00:50PM -0500, Waiman Long wrote:

> +static inline int __down_read_trylock(struct rw_semaphore *sem)
> +{
> + long tmp;
> +
> + while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> + if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> + tmp + RWSEM_ACTIVE_READ_BIAS)) {
> + return 1;
> + }
> + }
> + return 0;
> +}

So the orignal x86 implementation reads:

static inline bool __down_read_trylock(struct rw_semaphore *sem)
{
long result, tmp;
asm volatile("# beginning __down_read_trylock\n\t"
" mov %[count],%[result]\n\t"
"1:\n\t"
" mov %[result],%[tmp]\n\t"
" add %[inc],%[tmp]\n\t"
" jle 2f\n\t"
LOCK_PREFIX " cmpxchg %[tmp],%[count]\n\t"
" jnz 1b\n\t"
"2:\n\t"
"# ending __down_read_trylock\n\t"
: [count] "+m" (sem->count), [result] "=&a" (result),
[tmp] "=&r" (tmp)
: [inc] "i" (RWSEM_ACTIVE_READ_BIAS)
: "memory", "cc");
return result >= 0;
}

you replace that with:

int __down_read_trylock1(unsigned long *l)
{
long tmp;

while ((tmp = READ_ONCE(*l)) >= 0) {
if (tmp == cmpxchg(l, tmp, tmp + 1))
return 1;
}

return 0;
}

which generates:

0000000000000000 <__down_read_trylock1>:
0: eb 17 jmp 19 <__down_read_trylock1+0x19>
2: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
8: 48 8d 4a 01 lea 0x1(%rdx),%rcx
c: 48 89 d0 mov %rdx,%rax
f: f0 48 0f b1 0f lock cmpxchg %rcx,(%rdi)
14: 48 39 c2 cmp %rax,%rdx
17: 74 0f je 28 <__down_read_trylock1+0x28>
19: 48 8b 17 mov (%rdi),%rdx
1c: 48 85 d2 test %rdx,%rdx
1f: 79 e7 jns 8 <__down_read_trylock1+0x8>
21: 31 c0 xor %eax,%eax
23: c3 retq
24: 0f 1f 40 00 nopl 0x0(%rax)
28: b8 01 00 00 00 mov $0x1,%eax
2d: c3 retq

Which is clearly worse. Now we can write that as:

int __down_read_trylock2(unsigned long *l)
{
long tmp = READ_ONCE(*l);

while (tmp >= 0) {
if (try_cmpxchg(l, &tmp, tmp + 1))
return 1;
}

return 0;
}

which generates:

0000000000000030 <__down_read_trylock2>:
30: 48 8b 07 mov (%rdi),%rax
33: 48 85 c0 test %rax,%rax
36: 78 18 js 50 <__down_read_trylock2+0x20>
38: 48 8d 50 01 lea 0x1(%rax),%rdx
3c: f0 48 0f b1 17 lock cmpxchg %rdx,(%rdi)
41: 75 f0 jne 33 <__down_read_trylock2+0x3>
43: b8 01 00 00 00 mov $0x1,%eax
48: c3 retq
49: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
50: 31 c0 xor %eax,%eax
52: c3 retq

Which is a lot better; but not quite there yet.

I've tried quite a bit, but I can't seem to get GCC to generate the:

add $1,%rdx
jle

required; stuff like:

new = old + 1;
if (new <= 0)

generates:

lea 0x1(%rax),%rdx
test %rdx, %rdx
jle

Ah well, have fun :-)

[-- Attachment #2: dtl.c --]
[-- Type: text/x-csrc, Size: 4530 bytes --]

typedef unsigned char u8;
typedef unsigned short u16;
typedef unsigned int u32;
typedef unsigned long long u64;
typedef signed char s8;
typedef signed short s16;
typedef signed int s32;
typedef signed long long s64;
typedef _Bool bool;

# define CC_SET(c) "\n\t/* output condition code " #c "*/\n"
# define CC_OUT(c) "=@cc" #c

#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

extern void __cmpxchg_wrong_size(void);

#define __raw_cmpxchg(ptr, old, new, size, lock) \
({ \
__typeof__(*(ptr)) __ret; \
__typeof__(*(ptr)) __old = (old); \
__typeof__(*(ptr)) __new = (new); \
switch (size) { \
case 1: \
{ \
volatile u8 *__ptr = (volatile u8 *)(ptr); \
asm volatile(lock "cmpxchgb %2,%1" \
: "=a" (__ret), "+m" (*__ptr) \
: "q" (__new), "0" (__old) \
: "memory"); \
break; \
} \
case 2: \
{ \
volatile u16 *__ptr = (volatile u16 *)(ptr); \
asm volatile(lock "cmpxchgw %2,%1" \
: "=a" (__ret), "+m" (*__ptr) \
: "r" (__new), "0" (__old) \
: "memory"); \
break; \
} \
case 4: \
{ \
volatile u32 *__ptr = (volatile u32 *)(ptr); \
asm volatile(lock "cmpxchgl %2,%1" \
: "=a" (__ret), "+m" (*__ptr) \
: "r" (__new), "0" (__old) \
: "memory"); \
break; \
} \
case 8: \
{ \
volatile u64 *__ptr = (volatile u64 *)(ptr); \
asm volatile(lock "cmpxchgq %2,%1" \
: "=a" (__ret), "+m" (*__ptr) \
: "r" (__new), "0" (__old) \
: "memory"); \
break; \
} \
default: \
__cmpxchg_wrong_size(); \
} \
__ret; \
})

#define __cmpxchg(ptr, old, new, size) \
__raw_cmpxchg((ptr), (old), (new), (size), LOCK_PREFIX)

#define cmpxchg(ptr, old, new) \
__cmpxchg(ptr, old, new, sizeof(*(ptr)))

#define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \
({ \
bool success; \
__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
__typeof__(*(_ptr)) __old = *_old; \
__typeof__(*(_ptr)) __new = (_new); \
switch (size) { \
case 1: \
{ \
volatile u8 *__ptr = (volatile u8 *)(_ptr); \
asm volatile(lock "cmpxchgb %[new], %[ptr]" \
CC_SET(z) \
: CC_OUT(z) (success), \
[ptr] "+m" (*__ptr), \
[old] "+a" (__old) \
: [new] "q" (__new) \
: "memory"); \
break; \
} \
case 2: \
{ \
volatile u16 *__ptr = (volatile u16 *)(_ptr); \
asm volatile(lock "cmpxchgw %[new], %[ptr]" \
CC_SET(z) \
: CC_OUT(z) (success), \
[ptr] "+m" (*__ptr), \
[old] "+a" (__old) \
: [new] "r" (__new) \
: "memory"); \
break; \
} \
case 4: \
{ \
volatile u32 *__ptr = (volatile u32 *)(_ptr); \
asm volatile(lock "cmpxchgl %[new], %[ptr]" \
CC_SET(z) \
: CC_OUT(z) (success), \
[ptr] "+m" (*__ptr), \
[old] "+a" (__old) \
: [new] "r" (__new) \
: "memory"); \
break; \
} \
case 8: \
{ \
volatile u64 *__ptr = (volatile u64 *)(_ptr); \
asm volatile(lock "cmpxchgq %[new], %[ptr]" \
CC_SET(z) \
: CC_OUT(z) (success), \
[ptr] "+m" (*__ptr), \
[old] "+a" (__old) \
: [new] "r" (__new) \
: "memory"); \
break; \
} \
default: \
__cmpxchg_wrong_size(); \
} \
if (unlikely(!success)) \
*_old = __old; \
likely(success); \
})

#define LOCK_PREFIX "lock; "

#define __try_cmpxchg(ptr, pold, new, size) \
__raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX)

#define try_cmpxchg(ptr, pold, new) \
__try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))

#define READ_ONCE(x) (*(volatile typeof(x) *)(&x))

int __down_read_trylock1(unsigned long *l)
{
long tmp;

while ((tmp = READ_ONCE(*l)) >= 0) {
if (tmp == cmpxchg(l, tmp, tmp + 1))
return 1;
}

return 0;
}

int __down_read_trylock2(unsigned long *l)
{
long tmp = READ_ONCE(*l);

while (tmp >= 0) {
if (try_cmpxchg(l, &tmp, tmp + 1))
return 1;
}

return 0;
}

int __down_read_trylock3(unsigned long *l)
{
long new, old = READ_ONCE(*l);

for (;;) {
new = old + 1;
if (new <= 0)
return 0;
if (try_cmpxchg(l, &old, new))
return 1;
}
}