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

[Waiman Long <waiman.long@hp.com>]

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.

Cheers,
Longman