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SubjectRe: [PATCH][RFC] %pd - for printing dentry name
On Tue, Feb 02, 2010 at 07:09:08AM +0000, Al Viro wrote:
> 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.

K.

> > 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.

It is possible to move it back inline after a grace period, so that the
external name would be in use for only a few milliseconds after the
rename, but that of course adds more complexity.

> 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.

Indeed!!!

> 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.

Certainly sounds worth a try.

Thanx, Paul


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