netdev - Re: [PATCH v6 07/46] percpu_rwlock: Allow writers to be readers, and add lockdep annotations

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Message-ID: <5122575E.5030405@linux.vnet.ibm.com>
Date:	Mon, 18 Feb 2013 22:01:26 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	linux-doc@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org, mingo@...nel.org,
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	rostedt@...dmis.org, rjw@...k.pl, namhyung@...nel.org,
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	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 07/46] percpu_rwlock: Allow writers to be readers,
 and add lockdep annotations

On 02/18/2013 09:21 PM, Michel Lespinasse wrote:
> On Mon, Feb 18, 2013 at 8:39 PM, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> @@ -200,6 +217,16 @@ void percpu_write_lock_irqsave(struct percpu_rwlock *pcpu_rwlock,
>>
>>         smp_mb(); /* Complete the wait-for-readers, before taking the lock */
>>         write_lock_irqsave(&pcpu_rwlock->global_rwlock, *flags);
>> +
>> +       /*
>> +        * It is desirable to allow the writer to acquire the percpu-rwlock
>> +        * for read (if necessary), without deadlocking or getting complaints
>> +        * from lockdep. To achieve that, just increment the reader_refcnt of
>> +        * this CPU - that way, any attempt by the writer to acquire the
>> +        * percpu-rwlock for read, will get treated as a case of nested percpu
>> +        * reader, which is safe, from a locking perspective.
>> +        */
>> +       this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt);
> 
> I find this quite disgusting, but once again this may be because I
> don't like unfair recursive rwlocks.
> 

:-)

> In my opinion, the alternative of explicitly not taking the read lock
> when one already has the write lock sounds *much* nicer.

I don't seem to recall any strong reasons to do it this way, so I don't have
any strong opinions on doing it this way. But one of the things to note is that,
in the CPU Hotplug case, the readers are *way* more hotter than the writer.
So avoiding extra checks/'if' conditions/memory barriers in the reader-side
is very welcome. (If we slow down the read-side, we get a performance hit
even when *not* doing hotplug!). Considering this, the logic used in this
patchset seems better, IMHO.

Regards,
Srivatsa S. Bhat

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