netdev - Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend

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Message-ID: <1358883152.21576.55.camel@gandalf.local.home>
Date:	Tue, 22 Jan 2013 14:32:32 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	rjw@...k.pl, sbw@....edu, fweisbec@...il.com,
	linux@....linux.org.uk, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global
 reader-writer lock backend

On Tue, 2013-01-22 at 13:03 +0530, Srivatsa S. Bhat wrote:
> A straight-forward (and obvious) algorithm to implement Per-CPU Reader-Writer
> locks can also lead to too many deadlock possibilities which can make it very
> hard/impossible to use. This is explained in the example below, which helps
> justify the need for a different algorithm to implement flexible Per-CPU
> Reader-Writer locks.
> 
> We can use global rwlocks as shown below safely, without fear of deadlocks:
> 
> Readers:
> 
>          CPU 0                                CPU 1
>          ------                               ------
> 
> 1.    spin_lock(&random_lock);             read_lock(&my_rwlock);
> 
> 
> 2.    read_lock(&my_rwlock);               spin_lock(&random_lock);
> 
> 
> Writer:
> 
>          CPU 2:
>          ------
> 
>        write_lock(&my_rwlock);
> 

I thought global locks are now fair. That is, a reader will block if a
writer is waiting. Hence, the above should deadlock on the current
rwlock_t types.

We need to fix those locations (or better yet, remove all rwlocks ;-)

-- Steve


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