Re: [RFC] pdirops: vfs patch

[Jan Blunck]

Alex Tomas wrote:
+static inline struct semaphore * lock_sem(struct inode *dir, struct qstr *name)
+{
+       if (IS_PDIROPS(dir)) {
+               struct super_block *sb;
+               /* name->hash expected to be already calculated */
+               sb = dir->i_sb;
+               BUG_ON(sb->s_pdirops_sems == NULL);
+               return sb->s_pdirops_sems + name->hash % sb->s_pdirops_size;
+       }
+       return &dir->i_sem;
+}
+
+static inline void lock_dir(struct inode *dir, struct qstr *name)
+{
+       down(lock_sem(dir, name));
+}
+

@@ -1182,12 +1204,26 @@
 /*
  * p1 and p2 should be directories on the same fs.
  */
-struct dentry *lock_rename(struct dentry *p1, struct dentry *p2)
+struct dentry *lock_rename(struct dentry *p1, struct qstr *n1,
+				struct dentry *p2, struct qstr *n2)
 {
 	struct dentry *p;
 
 	if (p1 == p2) {
-		down(&p1->d_inode->i_sem);
+		if (IS_PDIROPS(p1->d_inode)) {
+			unsigned int h1, h2;
+			h1 = n1->hash % p1->d_inode->i_sb->s_pdirops_size;
+			h2 = n2->hash % p2->d_inode->i_sb->s_pdirops_size;
+			if (h1 < h2) {
+				lock_dir(p1->d_inode, n1);
+				lock_dir(p2->d_inode, n2);
+			} else if (h1 > h2) {
+				lock_dir(p2->d_inode, n2);
+				lock_dir(p1->d_inode, n1);
+			} else
+				lock_dir(p1->d_inode, n1);
+		} else
+			down(&p1->d_inode->i_sem);
 		return NULL;
 	}
 
@@ -1195,31 +1231,35 @@
 
 	for (p = p1; p->d_parent != p; p = p->d_parent) {
 		if (p->d_parent == p2) {
-			down(&p2->d_inode->i_sem);
-			down(&p1->d_inode->i_sem);
+			lock_dir(p2->d_inode, n2);
+			lock_dir(p1->d_inode, n1);
 			return p;
 		}
 	}
 
 	for (p = p2; p->d_parent != p; p = p->d_parent) {
 		if (p->d_parent == p1) {
-			down(&p1->d_inode->i_sem);
-			down(&p2->d_inode->i_sem);
+			lock_dir(p1->d_inode, n1);
+			lock_dir(p2->d_inode, n2);
 			return p;
 		}
 	}
 
-	down(&p1->d_inode->i_sem);
-	down(&p2->d_inode->i_sem);
+	lock_dir(p1->d_inode, n1);
+	lock_dir(p2->d_inode, n2);
 	return NULL;
 }
With luck you have s_pdirops_size (or 1024) different renames altering 
concurrently one directory inode. Therefore you need a lock protecting 
your filesystem data. This is basically the job done by i_sem. So in my 
opinion you only move "The Problem" from the VFS to the lowlevel 
filesystems. But then there is no need for i_sem or your s_pdirops_sems 
anymore.

Regards,
Jan
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