Re: [PATCH][RFC] %pd - for printing dentry name

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Feb 01, 2010 at 10:53:41PM -0800, Paul E. McKenney wrote:

> Here is an approximation that might inspire someone to come up with a
> real solution.
> 
> One approach would be to store the name length with the name, so that
> struct qstr loses the "len" field, and so that its "name" field points
> to a struct that has a "len" field followed by an array of const
> unsigned char.  That way, the name and length are closely associated.
> When you pick up a struct qstr's "name" pointer, you are guaranteed to
> get a length that matches the name.
> 
> Unfortunately:
> 
> o	In theory, this leaves the length of the dentry unchanged, but
> 	alignment is a problem on 64-bit systems.  Also, the long names
> 	gain an extra four bytes.

That one is not a big deal.

> o	If you get a pointer to the d_iname small-name field, rename
> 	might still change the name out from under you.  This could in
> 	theory be fixed by refusing to re-use the d_iname field until
> 	an RCU grace period had elapsed (using an external structure
> 	instead).  In practice, not sure if this is really a reasonable
> 	approach.

That, OTOH, is - most of dentries use inline name and external one is
really a rarely used fallback.  Making it a common case isn't nice.

There's another practical problem - a lot of code uses qstr fields and
patch will be painful; I couldn't care less about the out-of-tree code,
but it's a flagday change and in-tree patch size is not something to
sneeze at - I've been crawling through all that code for the last couple
of days and there's a lot of it.

Trying to play with seqlock-based solutions sounds more promising; I've
missed it completely and I'm half-asleep right now, so I'll try to take
a look at that after I get some sleep.