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

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Date:	Tue, 15 May 2007 09:19:37 +0200
From:	"Jan Blunck" <jblunck@...e.de>
To:	"Jan Engelhardt" <jengelh@...ux01.gwdg.de>, pbadari@...ibm.com
Cc:	"Bharata B Rao" <bharata@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"Jan Blunck" <j.blunck@...harburg.de>
Subject: Re: [RFC][PATCH 5/14] Introduce union stack

On 5/14/07, Jan Engelhardt <jengelh@...ux01.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
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