Re: c25aa432ff: aim9.dir_rtns_1.ops_per_sec -24.5% regression

[Al Viro]

On Wed, Sep 18, 2019 at 08:17:36AM +0800, kernel test robot wrote:
> Greeting,
> 
> FYI, we noticed a -24.5% regression of aim9.dir_rtns_1.ops_per_sec due to commit:
> 
> 
> commit: c25aa432ff56e179bf5414edff3aa430d2b260c0 ("Fix the locking in dcache_readdir() and friends")
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next.git master

... and that, folks, is ->d_lock on the parent.  I.e. the original reason for
trying to do the damn thing lockless.

I'd really like to do a lockless variant, properly.  First we'd need to sort
the misuses of d_subdirs/d_child, though - there are several outright bugs
in the mainline.  And right now it looks like it wants to grow a series of
cleanups in autofs.  Oh, well...