linux-kernel - Re: [PATCH 08/11] membarrier: Provide core serializing command (v2)

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Message-ID: <20180129190923.GP2249@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Jan 2018 20:09:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "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>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Greg Hackmann <ghackmann@...gle.com>,
        Will Deacon <will.deacon@....com>,
        David Sehr <sehr@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        x86 <x86@...nel.org>, linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 08/11] membarrier: Provide core serializing command (v2)

On Mon, Jan 29, 2018 at 06:36:05PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz@...radead.org wrote:

> > Aaah, its the case where we do not pass through switch_mm(), the partial
> > comment got to me. I only realized after reading the next patch.
> 
> Indeed, if we read the entire comment, it's made clear that this case is for
> when switch_mm is not invoked, where the current mm is changed without going
> through switch_mm(), when scheduling between uthread->kthread->uthread for
> instance.
> 
>         /*
>          * When transitioning from a kernel thread to a userspace
>          * thread, mmdrop()'s implicit full barrier is required by the
>          * membarrier system call, because the current active_mm can
>          * become the current mm without going through switch_mm().
>          * membarrier also requires a core serializing instruction
>          * before going back to user-space after storing to rq->curr.
>          */
> 
> Is there something I should improve in the wording of this added
> sentence to make it clearer ?

Can be improved I think, its got two unqualified "membarrier"s in and
its a bit mixed up. I'm having a major case of the mondays (brain just
won't start today), but maybe something like:

	When we switched through a kernel thread, the loop in
	membarrier_{private,global}_expedited() can have observed that
	kernel thread and not issued an IPI. We will also not pass
	through switch_mm(). Membarrier requires a barrier after writing
	rq->curr and returning to userspace, so provide them here:

	  - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED
	  - a sync_core for SYNC_CORE



Also I think changing the changlog to state where we need core-sync
would be good. Currently the x86 patch does that, but not this one,
while this introduces the feature.