linux-kernel - Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

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Message-ID: <20171005184609.44185ce1@gandalf.local.home>
Date:   Thu, 5 Oct 2017 18:46:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.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>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited
 private command

On Thu, 5 Oct 2017 22:19:15 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> AFAIU the scheduler rq->lock is held while preemption is disabled.
> synchronize_sched() is used here to ensure that all pre-existing
> preempt-off critical sections have completed.
> 
> So saying that we use synchronize_sched() to synchronize with rq->lock
> would be stretching the truth a bit. It's actually only true because the
> scheduler holding the rq->lock is surrounded by a preempt-off
> critical section.

Not only is preemption disabled (which is true for all spinlocks, at
least with non PREEMPT_RT), but the rq->lock must also only be taken
with interrupts disabled (for PREEMPT_RT as well). Because it can also
be taken in interrupt context.

-- Steve