Re: [PATCH 0/2 v2] dcache: get/release read lock inread_seqbegin_or_lock() & friend

[Linus Torvalds]

On Thu, Sep 12, 2013 at 9:38 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Sep 12, 2013 at 7:55 AM, Waiman Long <Waiman.Long@xxxxxx> wrote:
>> Change log
>> ----------
>> v1->v2:
>>   - Rename the new seqlock primitives to read_seqexcl_lock* and
>>     read_seqexcl_unlock*.
>
> Applied.

Btw, when I tried to benchmark this, I failed miserably.

Why?

If you do a threaded benchmark of "getcwd()", you end up spending all
your time in a spinlock anyway: get_fs_root_and_pwd() takes the
fs->lock to get the root/pwd.

Now, AIM7 probably uses processes, not threads, so you don't see this,
and maybe I shouldn't care. But looking at it, it annoys me
enormously, because the whole get_fs_root_and_pwd() is just stupid.

Putting it all under the RCU lock and then changing it to use
get_fs_root_and_pwd_rcu() that just uses the fs->seq sequence
read-lock looks absolutely trivial.

              Linus
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