linux-kernel - Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

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Date:	Mon, 10 Dec 2012 19:21:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	tj@...nel.org, tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, sbw@....edu, amit.kucheria@...aro.org,
	rjw@...k.pl, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light"
	atomic readers to prevent CPU offline

On 12/07, Oleg Nesterov wrote:
>
> On 12/06, Steven Rostedt wrote:
> >
> > You know reader locks can deadlock with each other, right? And this
> > isn't caught be lockdep yet. This is because rwlocks have been made to
> > be fair with writers. Before writers could be starved if a CPU always
> > let a reader in. Now if a writer is waiting, a reader will block behind
> > the writer. But this has introduced new issues with the kernel as
> > follows:
> >
> >
> >    CPU0			   CPU1	 	   CPU2		   CPU3
> >    ----			   ----		   ----		   ----
> > read_lock(A);
> > 			read_lock(B)
> > 					write_lock(A) <- block
> > 							write_lock(B) <- block
> > read_lock(B) <-block
> >
> > 			read_lock(A) <- block
> >
> > DEADLOCK!
>
> Really??? Oh I didn't know...
>
> Yes this was always true for rwsem, but rwlock_t?

Sorry, please ignore my email. I misread your email.

Oleg.

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