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Date: Sat, 7 Oct 2017 17:10:04 +0200 From: Andrea Parri <parri.andrea@...il.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, "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>, rostedt <rostedt@...dmis.org>, 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 Fri, Oct 06, 2017 at 10:32:19AM +0200, Peter Zijlstra 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. > > No, rq->lock is sufficient, note that rq->lock is a raw_spinlock_t which > implies !preempt. Yes, we also surround the rq->lock usage with a > slightly larger preempt_disable() section but that's not in fact > required for this. That's what it is, according to the current sources: we seemed to agree that a preempt-off critical section is what we rely on here and that the start of this critical section is not marked by that raw_spin_lock. Andrea
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