linux-kernel - Re: [RFC][PATCH 4/6] SRCU free VMAs

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

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@...radead.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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