VFS: delay the dentry name generation on sockets and pipes

1) Introduces a new method in 'struct dentry_operations'.  This method
   called d_dname() might be called from d_path() to build a pathname for
   special filesystems.  It is called without locks.

   Future patches (if we succeed in having one common dentry for all
   pipes/sockets) may need to change prototype of this method, but we now
   use : char *d_dname(struct dentry *dentry, char *buffer, int buflen);

2) Adds a dynamic_dname() helper function that eases d_dname() implementations

3) Defines d_dname method for sockets : No more sprintf() at socket
   creation.  This is delayed up to the moment someone does an access to
   /proc/pid/fd/...

4) Defines d_dname method for pipes : No more sprintf() at pipe
   creation.  This is delayed up to the moment someone does an access to
   /proc/pid/fd/...

A benchmark consisting of 1.000.000 calls to pipe()/close()/close() gives a
*nice* speedup on my Pentium(M) 1.6 Ghz :

3.090 s instead of 3.450 s

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Christoph Hellwig <hch@infradead.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/Locking

@@ -15,6 +15,7 @@ prototypes:
 	int (*d_delete)(struct dentry *);
 	void (*d_release)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
+	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 
 locking rules:
 	none have BKL
@@ -25,6 +26,7 @@ d_compare:	no		yes		no		no
 d_delete:	yes		no		yes		no
 d_release:	no		no		no		yes
 d_iput:		no		no		no		yes
+d_dname:	no		no		no		no
 
 --------------------------- inode_operations --------------------------- 
 prototypes:

Documentation/filesystems/vfs.txt

@@ -827,7 +827,7 @@ This describes how a filesystem can overload the standard dentry
 operations. Dentries and the dcache are the domain of the VFS and the
 individual filesystem implementations. Device drivers have no business
 here. These methods may be set to NULL, as they are either optional or
-the VFS uses a default. As of kernel 2.6.13, the following members are
+the VFS uses a default. As of kernel 2.6.22, the following members are
 defined:
 
 struct dentry_operations {
@@ -837,6 +837,7 @@ struct dentry_operations {
 	int (*d_delete)(struct dentry *);
 	void (*d_release)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
+	char *(*d_dname)(struct dentry *, char *, int);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -859,6 +860,26 @@ struct dentry_operations {
 	VFS calls iput(). If you define this method, you must call
 	iput() yourself
 
+  d_dname: called when the pathname of a dentry should be generated.
+	Usefull for some pseudo filesystems (sockfs, pipefs, ...) to delay
+	pathname generation. (Instead of doing it when dentry is created,
+	its done only when the path is needed.). Real filesystems probably
+	dont want to use it, because their dentries are present in global
+	dcache hash, so their hash should be an invariant. As no lock is
+	held, d_dname() should not try to modify the dentry itself, unless
+	appropriate SMP safety is used. CAUTION : d_path() logic is quite
+	tricky. The correct way to return for example "Hello" is to put it
+	at the end of the buffer, and returns a pointer to the first char.
+	dynamic_dname() helper function is provided to take care of this.
+
+Example :
+
+static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
+{
+	return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
+				dentry->d_inode->i_ino);
+}
+
 Each dentry has a pointer to its parent dentry, as well as a hash list
 of child dentries. Child dentries are basically like files in a
 directory.