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

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Date:	Mon, 11 Feb 2013 12:41:31 +0000
From:	David Howells <dhowells@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	dhowells@...hat.com, tglx@...utronix.de, peterz@...radead.org,
	tj@...nel.org, oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
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Subject: Re: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend

Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> wrote:

> 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);

The lock order on CPU 0 is unsafe if CPU2 can do:

	write_lock(&my_rwlock);
	spin_lock(&random_lock);

and on CPU 1 if CPU2 can do:

	spin_lock(&random_lock);
	write_lock(&my_rwlock);

I presume you were specifically excluding these situations?

David
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