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

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Message-ID: <Pine.LNX.4.64.1210181525010.32376@file.rdu.redhat.com>
Date:	Thu, 18 Oct 2012 15:28:56 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] brw_mutex: big read-write mutex



On Thu, 18 Oct 2012, Oleg Nesterov wrote:

> Ooooh. And I just noticed include/linux/percpu-rwsem.h which does
> something similar. Certainly it was not in my tree when I started
> this patch... percpu_down_write() doesn't allow multiple writers,
> but the main problem it uses msleep(1). It should not, I think.

synchronize_rcu() can sleep for hundred milliseconds, so msleep(1) is not 
a big problem.

> But. It seems that percpu_up_write() is equally wrong? Doesn't
> it need synchronize_rcu() before "p->locked = false" ?

Yes, it does ... and I sent patch for that to Linus.

> (add Mikulas)
> 
> Oleg.

Mikulas
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