linux-kernel - Re: [RFC please help] membarrier: Rewrite sync_core_before

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Message-ID: <20210105163731.GM3040@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Jan 2021 17:37:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Will Deacon <will@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        X86 ML <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [RFC please help] membarrier: Rewrite sync_core_before_usermode()

On Tue, Jan 05, 2021 at 08:20:51AM -0800, Andy Lutomirski wrote:
> >     Interestingly, the architecture recently added a control bit to remove
> >     this synchronisation from exception return, so if we set that then we'd
> >     have a problem with SYNC_CORE and adding an ISB would be necessary (and
> >     we could probable then make kernel->kernel returns cheaper, but I
> >     suspect we're relying on this implicit synchronisation in other places
> >     too).
> > 
> 
> Is ISB just a context synchronization event or does it do more?

IIRC it just the instruction sync (like power ISYNC).

> On x86, it’s very hard to tell that MFENCE does any more than LOCK,
> but it’s much slower.  And we have LFENCE, which, as documented,
> doesn’t appear to have any semantics at all.  (Or at least it didn’t
> before Spectre.)

AFAIU MFENCE is a completion barrier, while LOCK prefix is not. A bit
like ARM's DSB vs DMB.

It is for this reason that mb() is still MFENCE, while our smp_mb() is a
LOCK prefixed NO-OP.

And yes, LFENCE used to be poorly defined and it was sometimes
understood to be a completion barrier relative to prior LOADs, while it
is now a completion barrier for any prior instruction, and really should
be renamed to IFENCE.