linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Date:	Thu, 10 Oct 2013 17:36:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On 10/10, Peter Zijlstra wrote:
>
> That said, Oleg wants to use the same scheme for percpu_rwsem,

Yes, and later then (I hope) get_online_cpus() will be just
current->cpuhp_ref++ || percpu_down_read().

(just in case, we only need ->cpuhp_ref to ensure that the readers
 can't starve the writers as it currently possible. iow to block the
 new readers but ensure that the recursive get/down_read can't block)

And please look at sb_wait_write/sb_start_write. It uses the same
logic except:

	- it doesn't have the "true" fast-path for readers

	- its "slow" mode is slower than necessary

	- it is buggy (afaics)

> and I'm
> not sure his write-side (something in uprobes) wants to do the same.

surely not ;)

Oleg.

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