linux-kernel - Re: [RFC PATCH v3] membarrier: provide core serialization

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Date:   Mon, 9 Oct 2017 09:32:53 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Hans Boehm <hboehm@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        gromer <gromer@...gle.com>, Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Andy Lutomirski <luto@...nel.org>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        David Sehr <sehr@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>, ralf@...ux-mips.org
Subject: Re: [RFC PATCH v3] membarrier: provide core serialization

On Fri, Oct 06, 2017 at 11:08:25PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 06, 2017 at 08:57:56PM +0000, Mathieu Desnoyers wrote:
> > Hi Hans, 
> > 
> > I'm currently making sure the
> > MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED command makes its way into
> > the 4.14 kernel before the end of the release candidates.  Once that
> > is done, I plan to post a patch adding a new MEMBARRIER_FLAG_SYNC_CORE
> > flag for the 4.15 merge window. 
> > 
> > I have done a bit of research on the various architecture requirements
> > for core serialization.  Here are my findings so far about
> > instructions providing core serialization on the main architectures
> > supported by Linux. 
> > 
> > There are two places where we need it: in the interrupt handler for
> > the membarrier IPI, and between scheduler execution (which can change
> > the current "mm") and return to user-space. 
> > 
> > Please let me know if I missed anything. 
> > 
> > x86: iret, cpuid, wbinvd -> iret currently provides core serialization
> > when going back to userspace and at the end of the IPI. There are
> > plans to implement a return path without iret in the future, in which
> > case I would need to issue an explicit "cpuid" instruction
> > (sync_core()) in switch_mm() if the process is registered with
> > MEMBARRIER_FLAG_SYNC_CORE. 
> 
> I would much prefer setting a TIF flag that forces the IRET path instead
> of doing additional work in switch_mm().
> 
> > arm32: returning to user-space provides core serialization. Same at
> > the end of membarrier IPI (to be confirmed).  aarch64: ERET
> > instruction used when returning to user-space provides core sync. Same
> > at the end of membarrier IPI (to be confirmed). 
> 
> I thought Will already confirmed ERET did what we need, no?

Yes.

Will