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

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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