linux-kernel - Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc

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Date:   Tue, 14 May 2019 10:07:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     huangpei@...ngson.cn, Paul Burton <paul.burton@...s.com>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "andrea.parri@...rulasolutions.com" 
        <andrea.parri@...rulasolutions.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "paulmck@...ux.ibm.com" <paulmck@...ux.ibm.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Huacai Chen <chenhc@...ote.com>
Subject: Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

On Tue, May 14, 2019 at 9:56 AM Peter Zijlstra <peterz@...radead.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