proc: change proc_subdir_lock to a rwlock

The proc_subdir_lock spinlock is used to allow only one task to make
change to the proc directory structure as well as looking up information
in it.  However, the information lookup part can actually be entered by
more than one task as the pde_get() and pde_put() reference count update
calls in the critical sections are atomic increment and decrement
respectively and so are safe with concurrent updates.

The x86 architecture has already used qrwlock which is fair and other
architectures like ARM are in the process of switching to qrwlock.  So
unfairness shouldn't be a concern in that conversion.

This patch changed the proc_subdir_lock to a rwlock in order to enable
concurrent lookup. The following functions were modified to take a
write lock:
 - proc_register()
 - remove_proc_entry()
 - remove_proc_subtree()

The following functions were modified to take a read lock:
 - xlate_proc_name()
 - proc_lookup_de()
 - proc_readdir_de()

A parallel /proc filesystem search with the "find" command (1000 threads)
was run on a 4-socket Haswell-EX box (144 threads).  Before the patch, the
parallel search took about 39s.  After the patch, the parallel find took
only 25s, a saving of about 14s.

The micro-benchmark that I used was artificial, but it was used to
reproduce an exit hanging problem that I saw in real application.  In
fact, only allow one task to do a lookup seems too limiting to me.
Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Douglas Hatch <doug.hatch@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/generic.c

@@ -26,7 +26,7 @@
 
 #include "internal.h"
 
-static DEFINE_SPINLOCK(proc_subdir_lock);
+static DEFINE_RWLOCK(proc_subdir_lock);
 
 static int proc_match(unsigned int len, const char *name, struct proc_dir_entry *de)
 {
@@ -172,9 +172,9 @@ static int xlate_proc_name(const char *name, struct proc_dir_entry **ret,
 {
 	int rv;
 
-	spin_lock(&proc_subdir_lock);
+	read_lock(&proc_subdir_lock);
 	rv = __xlate_proc_name(name, ret, residual);
-	spin_unlock(&proc_subdir_lock);
+	read_unlock(&proc_subdir_lock);
 	return rv;
 }
 
@@ -231,11 +231,11 @@ struct dentry *proc_lookup_de(struct proc_dir_entry *de, struct inode *dir,
 {
 	struct inode *inode;
 
-	spin_lock(&proc_subdir_lock);
+	read_lock(&proc_subdir_lock);
 	de = pde_subdir_find(de, dentry->d_name.name, dentry->d_name.len);
 	if (de) {
 		pde_get(de);
-		spin_unlock(&proc_subdir_lock);
+		read_unlock(&proc_subdir_lock);
 		inode = proc_get_inode(dir->i_sb, de);
 		if (!inode)
 			return ERR_PTR(-ENOMEM);
@@ -243,7 +243,7 @@ struct dentry *proc_lookup_de(struct proc_dir_entry *de, struct inode *dir,
 		d_add(dentry, inode);
 		return NULL;
 	}
-	spin_unlock(&proc_subdir_lock);
+	read_unlock(&proc_subdir_lock);
 	return ERR_PTR(-ENOENT);
 }
 
@@ -270,12 +270,12 @@ int proc_readdir_de(struct proc_dir_entry *de, struct file *file,
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	spin_lock(&proc_subdir_lock);
+	read_lock(&proc_subdir_lock);
 	de = pde_subdir_first(de);
 	i = ctx->pos - 2;
 	for (;;) {
 		if (!de) {
-			spin_unlock(&proc_subdir_lock);
+			read_unlock(&proc_subdir_lock);
 			return 0;
 		}
 		if (!i)
@@ -287,19 +287,19 @@ int proc_readdir_de(struct proc_dir_entry *de, struct file *file,
 	do {
 		struct proc_dir_entry *next;
 		pde_get(de);
-		spin_unlock(&proc_subdir_lock);
+		read_unlock(&proc_subdir_lock);
 		if (!dir_emit(ctx, de->name, de->namelen,
 			    de->low_ino, de->mode >> 12)) {
 			pde_put(de);
 			return 0;
 		}
-		spin_lock(&proc_subdir_lock);
+		read_lock(&proc_subdir_lock);
 		ctx->pos++;
 		next = pde_subdir_next(de);
 		pde_put(de);
 		de = next;
 	} while (de);
-	spin_unlock(&proc_subdir_lock);
+	read_unlock(&proc_subdir_lock);
 	return 1;
 }
 
@@ -338,16 +338,16 @@ static int proc_register(struct proc_dir_entry * dir, struct proc_dir_entry * dp
 	if (ret)
 		return ret;
 
-	spin_lock(&proc_subdir_lock);
+	write_lock(&proc_subdir_lock);
 	dp->parent = dir;
 	if (pde_subdir_insert(dir, dp) == false) {
 		WARN(1, "proc_dir_entry '%s/%s' already registered\n",
 		     dir->name, dp->name);
-		spin_unlock(&proc_subdir_lock);
+		write_unlock(&proc_subdir_lock);
 		proc_free_inum(dp->low_ino);
 		return -EEXIST;
 	}
-	spin_unlock(&proc_subdir_lock);
+	write_unlock(&proc_subdir_lock);
 
 	return 0;
 }
@@ -549,9 +549,9 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 	const char *fn = name;
 	unsigned int len;
 
-	spin_lock(&proc_subdir_lock);
+	write_lock(&proc_subdir_lock);
 	if (__xlate_proc_name(name, &parent, &fn) != 0) {
-		spin_unlock(&proc_subdir_lock);
+		write_unlock(&proc_subdir_lock);
 		return;
 	}
 	len = strlen(fn);
@@ -559,7 +559,7 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 	de = pde_subdir_find(parent, fn, len);
 	if (de)
 		rb_erase(&de->subdir_node, &parent->subdir);
-	spin_unlock(&proc_subdir_lock);
+	write_unlock(&proc_subdir_lock);
 	if (!de) {
 		WARN(1, "name '%s'\n", name);
 		return;
@@ -583,16 +583,16 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 	const char *fn = name;
 	unsigned int len;
 
-	spin_lock(&proc_subdir_lock);
+	write_lock(&proc_subdir_lock);
 	if (__xlate_proc_name(name, &parent, &fn) != 0) {
-		spin_unlock(&proc_subdir_lock);
+		write_unlock(&proc_subdir_lock);
 		return -ENOENT;
 	}
 	len = strlen(fn);
 
 	root = pde_subdir_find(parent, fn, len);
 	if (!root) {
-		spin_unlock(&proc_subdir_lock);
+		write_unlock(&proc_subdir_lock);
 		return -ENOENT;
 	}
 	rb_erase(&root->subdir_node, &parent->subdir);
@@ -605,7 +605,7 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 			de = next;
 			continue;
 		}
-		spin_unlock(&proc_subdir_lock);
+		write_unlock(&proc_subdir_lock);
 
 		proc_entry_rundown(de);
 		next = de->parent;
@@ -616,7 +616,7 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 			break;
 		pde_put(de);
 
-		spin_lock(&proc_subdir_lock);
+		write_lock(&proc_subdir_lock);
 		de = next;
 	}
 	pde_put(root);