linux-kernel - Re: [PATCH 1/2] brw_mutex: big read-write mutex

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Message-ID: <20121019175737.GA1936@redhat.com>
Date:	Fri, 19 Oct 2012 19:57:37 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex

On 10/19, Peter Zijlstra wrote:
>
> But using preempt_{disable,enable} and using synchronize_sched() would
> be better (for PREEMPT_RCU) although it wouldn't fix anything
> fundamental.

BTW, I agree. I didn't even notice percpu-rwsem.h uses _rcu, not _sched.

> Fine goal, although somewhat arch specific. Also note that there's a
> relation between atomics and memory barriers, one isn't necessarily
> worse than the other, they all require synchronization of sorts.

As Paul pointed out, the fast path can avoid mb(). It is only needed
when "up_read" detects the writer.

Oleg.

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