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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 14 May 2019 10:07:53 -0700
Message-ID: <CAHk-=wgo_NxNYBSfSSGUV=CJPsz6nm_H6UnwsArBb-9GZ_sY_g@mail.gmail.com>
Subject: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     huangpei@loongson.cn, Paul Burton <paul.burton@mips.com>,
        "stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
        "akiyks@gmail.com" <akiyks@gmail.com>,
        "andrea.parri@amarulasolutions.com" 
        <andrea.parri@amarulasolutions.com>,
        "boqun.feng@gmail.com" <boqun.feng@gmail.com>,
        "dlustig@nvidia.com" <dlustig@nvidia.com>,
        "dhowells@redhat.com" <dhowells@redhat.com>,
        "j.alglave@ucl.ac.uk" <j.alglave@ucl.ac.uk>,
        "luc.maranget@inria.fr" <luc.maranget@inria.fr>,
        "npiggin@gmail.com" <npiggin@gmail.com>,
        "paulmck@linux.ibm.com" <paulmck@linux.ibm.com>,
        "will.deacon@arm.com" <will.deacon@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Huacai Chen <chenhc@lemote.com>
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On Tue, May 14, 2019 at 9:56 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Understood; the problem is that "*p++" is not good enough for local_t
> either (on load-store architectures), since it needs to be "atomic" wrt
> all other instructions on that CPU, most notably exceptions.

Right. But I don't think that's the issue here, since if it was then
it would be a problem even on UP.

And while the CPU-local ones want atomicity, they *shouldn't* have the
issue of another CPU modifying them, so even if you were to lose
exclusive ownership of the cacheline (because some other CPU is
reading your per-cpu data for statistics of whatever), the final end
result should be fine.

End result: I suspect ll/sc still works for cpu-local stuff without
any extra loongson hacks.

But I agree that it would be good to verify.

               Linus