Re: [RFC][PATCH 5/14] Introduce union stack

Jan Blunck
2007-05-15T07:19:49Z

On 5/14/07, Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
>
> >+static inline void union_lock(struct dentry *dentry)
> >+{
> >+ if (unlikely(dentry && dentry->d_union)) {
> >+ struct union_info *ui = dentry->d_union;
> >+
> >+ UM_DEBUG_LOCK("\"%s\" locking %p (count=%d)\n",
> >+ dentry->d_name.name, ui,
> >+ atomic_read(&ui->u_count));
> >+ __union_lock(dentry->d_union);
> >+ }
> >+}
> >+
> >+static inline void union_unlock(struct dentry *dentry)
> >+{
> >+ if (unlikely(dentry && dentry->d_union)) {
> >+ struct union_info *ui = dentry->d_union;
> >+
> >+ UM_DEBUG_LOCK("\"%s\" unlocking %p (count=%d)\n",
> >+ dentry->d_name.name, ui,
> >+ atomic_read(&ui->u_count));
> >+ __union_unlock(dentry->d_union);
> >+ }
> >+}
>
> Do we really need the unlikely()? d_union may be a new feature,
> but it may very well be possible that someone puts the bigger
> part of his/her files under a union. And when d_unions get
> stable, people will probably begin making their root filesystem
> unioned for livecds, and then unlikely() will rather be a
> likely penalty. My stance: just
> if (dentry != NULL && dentry->d_union != NULL)
> This also goes for union_trylock.

Good question. My intention was that since most of the union code
costs performance (stack traversal, readdir) I optimize for the normal
(not unified) case.

> >+static inline int union_trylock(struct dentry *dentry)
> >+{
> >+ int locked = 1;
> >+
> >+ if (unlikely(dentry && dentry->d_union)) {
> >+ UM_DEBUG_LOCK("\"%s\" try locking %p (count=%d)\n",
> >+ dentry->d_name.name, dentry->d_union,
> >+ atomic_read(&dentry->d_union->u_count));
> >+ BUG_ON(!atomic_read(&dentry->d_union->u_count));
> >+ locked = mutex_trylock(&dentry->d_union->u_mutex);
> >+ UM_DEBUG_LOCK("\"%s\" trylock %p %s\n", dentry->d_name.name,
> >+ dentry->d_union,
> >+ locked ? "succeeded" : "failed");
> >+ }
> >+ return (locked ? 1 : 0);
> >+}
>
> return locked ? 1 : 0
> or even
> return !!locked;
> or since we're just passing up from mutex_trylock:
> return locked;
> ?

Ahh, this seems to be a left-over of the semaphore -> mutex conversion.

> >+/*
> >+ * This is a *I can't get no sleep* helper
>
> More commonly known as "insomnia". :)
>

:)

Before I forget this: thank you (and Badari) for reviewing the patches!

Cheers,
Jan