On 01/22/2012 12:39 PM, Al Viro wrote:

On Sun, Jan 22, 2012 at 12:31:31PM -0800, wangdi wrote:

quoted

We actually already implemented this for ext4, and we saw a lot performance improvement(at least 30% improvements for open/create in a single directory)for lustre stack,
but we want to make this improvement accessible through the VFS. Probably XFS and Btrfs could also benefit from this.

You do realize that i_mutex locking is relied upon for protection of a lot
of stuff besides the obvious (i.e. on-disk directory contents)?
I'm not saying that it's hopeless, but it's highly non-trivial; the things
like rmdir/mount races, access to ->d_parent/->d_name in a lot of code,
etc. need to be taken care of and it is a _lot_ of code review to deal
with - just to verify the correctness of such changes.

Yes, I agree it is non-trivial change here. What I want to say is that 
i_mutex lock might be too big in some cases, and it just serializes 
everything. So it might be useful  if we could refine this lock a bit.  
For example we can define this lock with several modes, (read, write, 
current read, current write, exclusive etc), and different code can get 
the lock with different mode as required, which might bring us some 
concurrency.

Thanks
WangDi