linux-kernel - Re: [PATCH v9 05/15] arm64: barrier: add support for smp_cond_relaxed

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Message-ID: <878qsn9tyv.fsf@oracle.com>
Date: Tue, 10 Dec 2024 12:14:00 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Will Deacon <will@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-pm@...r.kernel.org,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        catalin.marinas@....com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, pbonzini@...hat.com, vkuznets@...hat.com,
        rafael@...nel.org, daniel.lezcano@...aro.org, peterz@...radead.org,
        arnd@...db.de, lenb@...nel.org, mark.rutland@....com,
        harisokn@...zon.com, mtosatti@...hat.com, sudeep.holla@....com,
        cl@...two.org, maz@...nel.org, misono.tomohiro@...itsu.com,
        maobibo@...ngson.cn, zhenglifeng1@...wei.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v9 05/15] arm64: barrier: add support for
 smp_cond_relaxed_timeout()


Will Deacon <will@...nel.org> writes:

> On Thu, Nov 07, 2024 at 11:08:08AM -0800, Ankur Arora wrote:
>> Support a waited variant of polling on a conditional variable
>> via smp_cond_relaxed_timeout().
>>
>> This uses the __cmpwait_relaxed() primitive to do the actual
>> waiting, when the wait can be guaranteed to not block forever
>> (in case there are no stores to the waited for cacheline.)
>> For this we depend on the availability of the event-stream.
>>
>> For cases when the event-stream is unavailable, we fallback to
>> a spin-waited implementation which is identical to the generic
>> variant.
>>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>  arch/arm64/include/asm/barrier.h | 54 ++++++++++++++++++++++++++++++++
>>  1 file changed, 54 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index 1ca947d5c939..ab2515ecd6ca 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -216,6 +216,60 @@ do {									\
>>  	(typeof(*ptr))VAL;						\
>>  })
>>
>> +#define __smp_cond_load_timeout_spin(ptr, cond_expr,			\
>> +				     time_expr_ns, time_limit_ns)	\
>> +({									\
>> +	typeof(ptr) __PTR = (ptr);					\
>> +	__unqual_scalar_typeof(*ptr) VAL;				\
>> +	unsigned int __count = 0;					\
>> +	for (;;) {							\
>> +		VAL = READ_ONCE(*__PTR);				\
>> +		if (cond_expr)						\
>> +			break;						\
>> +		cpu_relax();						\
>> +		if (__count++ < smp_cond_time_check_count)		\
>> +			continue;					\
>> +		if ((time_expr_ns) >= time_limit_ns)			\
>> +			break;						\
>> +		__count = 0;						\
>> +	}								\
>> +	(typeof(*ptr))VAL;						\
>> +})
>
> This is a carbon-copy of the asm-generic timeout implementation. Please
> can you avoid duplicating that in the arch code?

Yeah I realized a bit late that I could avoid the duplication quite
simply. Will fix.

Thanks

--
ankur