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Date: Tue, 21 Oct 2014 10:22:03 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC][PATCH 4/6] SRCU free VMAs
Message-ID: <20141021082203.GL23531@worktop.programming.kicks-ass.net>
References: <20141020215633.717315139@infradead.org>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>, Minchan Kim <minchan@kernel.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Al Viro <viro@zeniv.linux.org.uk>, Lai Jiangshan <laijs@cn.fujitsu.com>, Davidlohr Bueso <dave@stgolabs.net>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>

On Mon, Oct 20, 2014 at 04:41:45PM -0700, Linus Torvalds wrote:
> On Mon, Oct 20, 2014 at 2:56 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > Manage the VMAs with SRCU such that we can do a lockless VMA lookup.
> 
> Can you explain why srcu, and not plain regular rcu?
> 
> Especially as you then *note* some of the problems srcu can have.
> Making it regular rcu would also seem to make it possible to make the
> seqlock be just a seqcount, no?

Ah, the reason I did the seqlock is because the read side will spin-wait
for &1 to go away. If the write side is preemptible that's horrid. I
used seqlock because that takes a lock (and thus disables preemption) on
the write side, but I could equally have done:

	preempt_disable();
	write_seqcount_begin();

	...

	write_seqcount_end();
	preempt_disable();

Since the lock is indeed superfluous, we're already fully serialized by
mmap_sem in this path.

Using regular RCU isn't sufficient, because of PREEMPT_RCU.

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