linux-kernel - Re: processes hung after sys_renameat, and 'missing' processes

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Date:	Fri, 8 Jun 2012 01:36:04 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org,
	"J. Bruce Fields" <bfields@...hat.com>,
	Sage Weil <sage@...dream.net>
Subject: Re: processes hung after sys_renameat, and 'missing' processes

On Thu, Jun 07, 2012 at 04:57:13PM -0700, Linus Torvalds wrote:

> Any per-filesystem mutex should do, so if sysfs always holds the
> sysfs_mutex - and never allows user-initiated renames - it should be
> safe.

Frankly, I would very much prefer to have the same locking rules wherever
possible.  The locking system is already overcomplicated and making its
analysis fs-dependent as well... <shudder>  Sure, we can do that, and that
might even work, until we find out that some piece of code that started
as a helper to some function never called on sysfs dentries had been
reused on the path that *is* reachable on sysfs.  At which point we are
suddenly in trouble.

I wouldn't be bothered so much if the overall picture had been simpler;
unfortunately, it isn't.

Eric, how about this - if nothing else, that makes code in there simpler
and less dependent on details of VFS guts:

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index e6bb9b2..5579826 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -363,7 +363,7 @@ static void sysfs_dentry_iput(struct dentry *dentry, struct inode *inode)
 	iput(inode);
 }
 
-static const struct dentry_operations sysfs_dentry_ops = {
+const struct dentry_operations sysfs_dentry_ops = {
 	.d_revalidate	= sysfs_dentry_revalidate,
 	.d_delete	= sysfs_dentry_delete,
 	.d_iput		= sysfs_dentry_iput,
@@ -795,16 +795,8 @@ static struct dentry * sysfs_lookup(struct inode *dir, struct dentry *dentry,
 	}
 
 	/* instantiate and hash dentry */
-	ret = d_find_alias(inode);
-	if (!ret) {
-		d_set_d_op(dentry, &sysfs_dentry_ops);
-		dentry->d_fsdata = sysfs_get(sd);
-		d_add(dentry, inode);
-	} else {
-		d_move(ret, dentry);
-		iput(inode);
-	}
-
+	dentry->d_fsdata = sysfs_get(sd);
+	ret = d_materialise_unique(dentry, inode);
  out_unlock:
 	mutex_unlock(&sysfs_mutex);
 	return ret;
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 52c3bdb..c15a7a3 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -68,6 +68,7 @@ static int sysfs_fill_super(struct super_block *sb, void *data, int silent)
 	}
 	root->d_fsdata = &sysfs_root;
 	sb->s_root = root;
+	sb->s_d_op = &sysfs_dentry_ops;
 	return 0;
 }
 
diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index 661a963..d73c093 100644
--- a/fs/sysfs/sysfs.h
+++ b/fs/sysfs/sysfs.h
@@ -157,6 +157,7 @@ extern struct kmem_cache *sysfs_dir_cachep;
  */
 extern struct mutex sysfs_mutex;
 extern spinlock_t sysfs_assoc_lock;
+extern const struct dentry_operations sysfs_dentry_ops;
 
 extern const struct file_operations sysfs_dir_operations;
 extern const struct inode_operations sysfs_dir_inode_operations;
--
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