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Message-ID: <041c83e6-1ca5-4953-bcec-03571135165d@efficios.com>
Date: Wed, 29 Nov 2023 16:32:43 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: paulmck@...nel.org, palmer@...belt.com, paul.walmsley@...ive.com,
aou@...s.berkeley.edu, mmaas@...gle.com, hboehm@...gle.com,
striker@...ibm.com, charlie@...osinc.com, rehn@...osinc.com,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command
On 2023-11-29 16:25, Andrea Parri wrote:
>>> As regards the Fixes: tag, I guess it boils down to what we want or we
>>> need to take for commit "riscv: Support membarrier private cmd". :-)
>>
>> I'm not seeing this commit in the Linux master branch, am I missing
>> something ?
>
> I don't think you're missing something: I was wondering "what/where is
> this commit"? Sorry for the confusion.
>
>
>>> FWIW, a quick git-log search confirmed that MEMBARRIER has been around
>>> for quite some time in the RISC-V world (though I'm not familiar with
>>> any of its mainstream uses): commit 1464d00b27b2 says (at least) since
>>> 93917ad50972 ("RISC-V: Add support for restartable sequence"). I am
>>> currently inclined to pick the latter commit (and check it w/ Palmer),
>>> but other suggestions are welcome.
>>
>> Supporting membarrier private expedited is not optional since Linux 4.14:
>>
>> see kernel/sched/core.c:
>>
>> membarrier_switch_mm(rq, prev->active_mm, next->mm);
>> /*
>> * sys_membarrier() requires an smp_mb() between setting
>> * rq->curr / membarrier_switch_mm() and returning to userspace.
>> *
>> * The below provides this either through switch_mm(), or in
>> * case 'prev->active_mm == next->mm' through
>> * finish_task_switch()'s mmdrop().
>> */
>> switch_mm_irqs_off(prev->active_mm, next->mm, next);
>>
>> Failure to provide the required barrier is a bug in the architecture's
>> switch_mm implementation when CONFIG_MEMBARRIER=y.
>>
>> We should probably introduce a new
>> Documentation/features/sched/membarrier/arch-support.txt
>> to clarify this requirement.
>>
>> Userspace code such as liburcu [1] heavily relies on membarrier private
>> expedited (when available) to speed up RCU read-side critical sections.
>> Various DNS servers, including BIND 9, use liburcu.
>
> Thanks for the information.
>
> So I should probably stick to 93917ad50972, which apparently selected
> CONFIG_MEMBARRIER on RISC-V, for the Fixes: tag in question.
I think it goes further than that, because you can explicitly
CONFIG_MEMBARRIER=y, see init/Kconfig:
config MEMBARRIER
bool "Enable membarrier() system call" if EXPERT
default y
help
Enable the membarrier() system call that allows issuing memory
barriers across all running threads, which can be used to distribute
the cost of user-space memory barriers asymmetrically by transforming
pairs of memory barriers into pairs consisting of membarrier() and a
compiler barrier.
If unsure, say Y.
Before 1464d00b27b2, riscv just happened to set it to =n in the defconfig.
I suspect the initial port of riscv merged after v4.14 was already broken.
> I'll look into adding the membarrier feature you mention (as a final/
> follow-up patch), unless you or someone else want to take care of it.
I'll be happy to review it :)
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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