Re: [PATCH] proc: change proc_subdir_lock to a rwlock

Eric W. Biederman
2015-08-03T18:10:09Z

Waiman Long <waiman.long@hp.com> writes:

> On 07/30/2015 10:16 PM, Waiman Long wrote:
>> On 07/29/2015 06:21 PM, Eric W. Biederman wrote:
>>> Two quick questions.
>>>
>>> - What motivates this work? Are you seeing lots of
>>> parallel reads on proc?
>>
>> The micro-benchmark that I used was artificial, but it was used to reproduce
>> an exit hanging problem that I saw in real application. In fact, only allow
>> one task to do a lookup seems too limiting to me.
>>> - Why not rcu? Additions and removal of proc generic
>>> files is very rare. Conversion to rcu for reads should
>>> perform better and not take much more work.
>>
>> RCU is harder to verify its correctness, whereas rwlock is easier to use and
>> understand. If it is really a performance critical path where every extra bit
>> of performance counts, I will certainly think RCU may be the right
>> choice. However, in this particular case, I don't think using RCU will give
>> any noticeable performance gain compared with a rwlock.
>
> One more thing, RCU is typically used with linked list. It is not easy to use
> RCU with rbtree and may require major changes to the code.
>
> Another alternative is to use seqlock + RCU, but it will still need more code
> changes than rwlock.

I had forgotten we had switch proc directories to rbtrees. So on that
note.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

Eric