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

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Date:   Thu, 25 Apr 2019 09:12:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     huangpei@...ngson.cn
Cc:     Paul Burton <paul.burton@...s.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "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 Wed, Apr 24, 2019 at 9:59 PM <huangpei@...ngson.cn> wrote:
>
> In one word， the bug only affect local cpu‘s ll/sc operation, and affect MP system.
>
> If local_t is only ll/sc manipulated by current CPU， then no need fix it.

As to the whole MP vs UP issue:

Is this "guaranteed no problem on UP" true even in the presence of
DMA? I'm _hoping_ some MIPS chips are starting to be coherent with DMA
(but maybe DMA never participates in any coherency traffic that could
trigger the bug even if the DMA _were_ to be coherent?).

Also, as Peter mentioned, we do depend on ll/sc also reacting to
interrupts - again including on UP systems, of course. I assume that's
always handled correctly, and that an interrupt will set the right
state that the SC will not succeed?

Finally, it worries me a bit that the loongson_llsc_mb() things are
all at the C level, so who knows what compiler behavior you'll see
between the ll/sc inline asm, and the loongson_llsc_mb() generation.
Hopefully not a lot (and presumably mostly just stack reloads etc that
probably don't change any cache state), but I do wonder if the
loongson quirk should be *inside* the asm rather than separate.

                  Linus