Re: [PATCH] arm64: atomics: Fix the issue on xchg when switch to atomic instruction

[Shaokun Zhang <zhangshaokun@hisilicon.com>]

Hi Will,

On 2020/5/6 18:44, Will Deacon wrote:
> On Wed, May 06, 2020 at 06:39:16PM +0800, Shaokun Zhang wrote:
>> Apologies for my noise, you are right and it's my mistake.
> 
> No need to apologise, but thanks for letting me know.
> 
>> On 2020/5/6 15:53, Will Deacon wrote:
>>> On Wed, May 06, 2020 at 03:00:39PM +0800, Shaokun Zhang wrote:
>>>> On 2020/5/5 17:15, Will Deacon wrote:
>>>>> On Tue, May 05, 2020 at 05:02:35PM +0800, Shaokun Zhang wrote:
>>>>>> From: Yuqi Jin <jinyuqi@huawei.com>
>>>>>>
>>>>>> Since commit addfc38672c7 ("arm64: atomics: avoid out-of-line ll/sc atomics"),
>>>>>> it has provided inline implementations of both LSE and ll/sc and used a static
>>>>>> key to select between them, which allows the compiler to generate better
>>>>>> atomics code.
>>>>>> However, xchg still uses the original method which would fail to switch to
>>>>>> the atomic instruction correctly, Let's fix this issue.
>>>>>
>>>>> Please can you elaborate on the failure mode? The current code looks alright
>>>>
>>>> When enable CONFIG_ARM64_LSE_ATOMICS, xchg is failed to switch to swp instruction
>>>> or dynamic replacement instructions are not seen.
>>>>
>>>> We do some tests on the copy of xchg_tail,:
>>>> u32 xchg_tail_my(struct qspinlock *lock, u32 tail)
>>>> {
>>>>         return (u32)xchg_relaxed(&lock->tail,
>>>>                                  tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
>>>> }
>>>> and the asm code is as follows:
>>>>
>>>> ffff80001015b050 <xchg_tail_my>:
>>>> ffff80001015b050:       a9be7bfd        stp     x29, x30, [sp, #-32]!
>>>> ffff80001015b054:       910003fd        mov     x29, sp
>>>> ffff80001015b058:       a90153f3        stp     x19, x20, [sp, #16]
>>>> ffff80001015b05c:       2a0103f3        mov     w19, w1
>>>> ffff80001015b060:       aa0003f4        mov     x20, x0
>>>> ffff80001015b064:       aa1e03e0        mov     x0, x30
>>>> ffff80001015b068:       97fd07ee        bl      ffff80001009d020 <_mcount>
>>>> ffff80001015b06c:       53107e61        lsr     w1, w19, #16
>>>> ffff80001015b070:       91000a83        add     x3, x20, #0x2
>>>> ffff80001015b074:       f9800071        prfm    pstl1strm, [x3]
>>>> ffff80001015b078:       485f7c60        ldxrh   w0, [x3]
>>>> ffff80001015b07c:       48027c61        stxrh   w2, w1, [x3]
>>>> ffff80001015b080:       35ffffc2        cbnz    w2, ffff80001015b078 <xchg_tail_my+0x28>
>>>> ffff80001015b084:       53103c00        lsl     w0, w0, #16
>>>> ffff80001015b088:       a94153f3        ldp     x19, x20, [sp, #16]
>>>> ffff80001015b08c:       a8c27bfd        ldp     x29, x30, [sp], #32
>>>> ffff80001015b090:       d65f03c0        ret
>>>
>>> This should get patched at runtime, but you're saying that's not happening?
>>>
>>
>> My mistake, I didn't check the runtime carefully.
> 
> Good to hear there's not a bug, but if you see a performance benefit from
> using the static-key for xchg() then I'd obviously be open to changing it

Thanks your reply, if I follow the two methods correctly, static-key will
not consume '__nops(3)', others are the same.

I will run some tests to check the performance  ;-)

Thanks,
Shaokun


> over as well.
> 
> Thanks,
> 
> Will
> 
> .
> 


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