netdev - Re: [PATCH v5 06/45] percpu_rwlock: Allow writers to be readers, and add lockdep annotations

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Date:	Mon, 11 Feb 2013 01:02:05 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, rusty@...tcorp.com.au, mingo@...nel.org,
	akpm@...ux-foundation.org, namhyung@...nel.org,
	rostedt@...dmis.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 06/45] percpu_rwlock: Allow writers to be readers,
 and add lockdep annotations

On 02/09/2013 05:17 AM, Paul E. McKenney wrote:
> On Tue, Jan 22, 2013 at 01:04:23PM +0530, Srivatsa S. Bhat wrote:
>> CPU hotplug (which will be the first user of per-CPU rwlocks) has a special
>> requirement with respect to locking: the writer, after acquiring the per-CPU
>> rwlock for write, must be allowed to take the same lock for read, without
>> deadlocking and without getting complaints from lockdep. In comparison, this
>> is similar to what get_online_cpus()/put_online_cpus() does today: it allows
>> a hotplug writer (who holds the cpu_hotplug.lock mutex) to invoke it without
>> locking issues, because it silently returns if the caller is the hotplug
>> writer itself.
>>
>> This can be easily achieved with per-CPU rwlocks as well (even without a
>> "is this a writer?" check) by incrementing the per-CPU refcount of the writer
>> immediately after taking the global rwlock for write, and then decrementing
>> the per-CPU refcount before releasing the global rwlock.
>> This ensures that any reader that comes along on that CPU while the writer is
>> active (on that same CPU), notices the non-zero value of the nested counter
>> and assumes that it is a nested read-side critical section and proceeds by
>> just incrementing the refcount. Thus we prevent the reader from taking the
>> global rwlock for read, which prevents the writer from deadlocking itself.
>>
>> Add that support and teach lockdep about this special locking scheme so
>> that it knows that this sort of usage is valid. Also add the required lockdep
>> annotations to enable it to detect common locking problems with per-CPU
>> rwlocks.
> 
> Very nice!  The write-side interrupt disabling ensures that the task
> stays on CPU, as required.
> 
> One request: Could we please have a comment explaining the reasons for
> the writer incrementing and decrementing the reader reference count?
> 
> It looked really really strange to me until I came back and read the
> commit log.  ;-)
> 

Sure :-)

Regards,
Srivatsa S. Bhat

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