linux-kernel - Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command

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Date:   Wed, 06 Dec 2023 05:05:48 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     parri.andrea@...il.com
CC:     mathieu.desnoyers@...icios.com, paulmck@...nel.org,
        Paul Walmsley <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 Wed, 29 Nov 2023 14:43:34 PST (-0800), parri.andrea@...il.com wrote:
>> > 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 see.  Oh well, guess I'll have to leave this up to the maintainers then
> (I believe I've never managed to build riscv that far), Palmer?

I see

$ git grep "config MEMBARRIER" fab957c11efe2f405e08b9f0d080524bc2631428
fab957c11efe2f405e08b9f0d080524bc2631428:init/Kconfig:config MEMBARRIER

so IMO this is just one of those forever bugs.  So I'd lean towards

    Fixes: fab957c11efe ("RISC-V: Atomic and Locking Code")

(or anything in that original patch set).  It's not that big of a 
backport, so I think it's safe enough?

>> > 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 :)
>
> Sweet!  :-)
>
>   Andrea