[PATCH 1/3] smb/client: don't unhash and rehash to prevent new opens.

[NeilBrown]

From: NeilBrown <neil@xxxxxxxxxx>

smb/client needs to block new opens of the target of unlink and rename
while the operation is progressing.  This stablises d_count() and allows
a determination of whether a "silly-rename" is required.

It currently unhashes the dentry which will cause lookup to block on
the parent directory i_rwsem.  Proposed changes to locking will cause
this approach to stop working as the exclusivity will be provided for
the dentry only, and only while it is hashed.

So we introduce a new machanism similar to that used by nfs.
DCACHE_PRIVATE (given the name DCACHE_BLOCKED) is set when lookups need
to be blocked.  ->d_revalidate checks for this and blocks.  This might
still allow d_count() to increment, but once it has been tested as 1,
there can be no new opens completed.

Unlike unhash which does not need to be reverted on error, and which
must not be reverted on a successful d_move, blocking of opens must
always be reverted.  So we don't block the open until after the last
early "return", and we always unblock on the final "return".

Strictly speaking it is only necessary to block opens before the value
of d_count() is tested under ->d_lock. But we block as early as possible
to discourage new opens from starting.  As open is path-based in cifs a
concurrent rename can be confused a rename of one other the paths.  Once
cifs_open() has generated the full_path it doesn't hold any locks to
prevent a rename from making that path invalid.

The important details of the interlock between __cifs_unlink (which both
unlink and rename use) and "open" are that __cifs_unlink() sets
DCACHE_BLOCKED  *before* testing d_count() which is
in a d_lock locked region,  and cifs_d_revalidate() tests the bit *after*
->d_lock is taken by e.g. __d_lookup() to increment d_count().
Thus ->d_lock provide serialization between the set and the test
even though the test isn't in a locked region.
(The fact that cifs_d_revalidate() always returned -ECHILD when
 LOOKUP_RCU is important for this sequencing to work as it ensures
 d_revalidate() is called *after* d_count() is incremented).

Signed-off-by: NeilBrown <neil@xxxxxxxxxx>
---
 fs/smb/client/cifsfs.h |  8 +++++
 fs/smb/client/dir.c    |  3 ++
 fs/smb/client/inode.c  | 68 ++++++++++++++++++++++++------------------
 3 files changed, 50 insertions(+), 29 deletions(-)

diff --git a/fs/smb/client/cifsfs.h b/fs/smb/client/cifsfs.h
index 0c85daa8386e..bcc0fb5c2c9b 100644
--- a/fs/smb/client/cifsfs.h
+++ b/fs/smb/client/cifsfs.h
@@ -42,6 +42,14 @@ static inline unsigned long cifs_get_time(struct dentry *dentry)
 	return (unsigned long) dentry->d_fsdata;
 }
 
+/*
+ * This is set to block d_revalidate on a dentry that is being removed -
+ * the target of unlink or rename.  This causes any open attempt to
+ * block.  There may be existing opens but they can be detected by
+ * checking d_count() under ->d_lock.
+ */
+#define DCACHE_BLOCKED DCACHE_PRIVATE
+
 extern struct file_system_type cifs_fs_type, smb3_fs_type;
 extern const struct address_space_operations cifs_addr_ops;
 extern const struct address_space_operations cifs_addr_ops_smallbuf;
diff --git a/fs/smb/client/dir.c b/fs/smb/client/dir.c
index 6fa6d48fdfd3..88fff65320f5 100644
--- a/fs/smb/client/dir.c
+++ b/fs/smb/client/dir.c
@@ -872,6 +872,9 @@ cifs_d_revalidate(struct inode *dir, const struct qstr *name,
 	if (flags & LOOKUP_RCU)
 		return -ECHILD;
 
+	/* Wait for pending rename/unlink */
+	wait_var_event(&direntry->d_flags, !(direntry->d_flags & DCACHE_BLOCKED));
+
 	if (d_really_is_positive(direntry)) {
 		int rc;
 		struct inode *inode = d_inode(direntry);
diff --git a/fs/smb/client/inode.c b/fs/smb/client/inode.c
index 12ed8db10e00..4d34c5622cd4 100644
--- a/fs/smb/client/inode.c
+++ b/fs/smb/client/inode.c
@@ -1967,24 +1967,31 @@ static int __cifs_unlink(struct inode *dir, struct dentry *dentry, bool sillyren
 	__u32 dosattr = 0, origattr = 0;
 	struct TCP_Server_Info *server;
 	struct iattr *attrs = NULL;
-	bool rehash = false;
+	bool unblock = false;
 
 	cifs_dbg(FYI, "cifs_unlink, dir=0x%p, dentry=0x%p\n", dir, dentry);
 
 	if (unlikely(cifs_forced_shutdown(cifs_sb)))
 		return smb_EIO(smb_eio_trace_forced_shutdown);
 
-	/* Unhash dentry in advance to prevent any concurrent opens */
-	spin_lock(&dentry->d_lock);
-	if (!d_unhashed(dentry)) {
-		__d_drop(dentry);
-		rehash = true;
-	}
-	spin_unlock(&dentry->d_lock);
-
 	tlink = cifs_sb_tlink(cifs_sb);
 	if (IS_ERR(tlink))
 		return PTR_ERR(tlink);
+
+	/* opens might already be blocked by rename */
+	if (!(dentry->d_flags & DCACHE_BLOCKED)) {
+		/*
+		 * Block opens - and all lookups that involve d_revalidate.
+		 * This guarantees that if another thread tries to open(), it
+		 * will either block, or will increment d_count()
+		 * before we test it below.
+		 */
+		spin_lock(&dentry->d_lock);
+		dentry->d_flags |= DCACHE_BLOCKED;
+		spin_unlock(&dentry->d_lock);
+		unblock = true;
+	}
+
 	tcon = tlink_tcon(tlink);
 	server = tcon->ses->server;
 
@@ -2107,8 +2114,13 @@ static int __cifs_unlink(struct inode *dir, struct dentry *dentry, bool sillyren
 	kfree(attrs);
 	free_xid(xid);
 	cifs_put_tlink(tlink);
-	if (rehash)
-		d_rehash(dentry);
+	/* Allow lookups/opens */
+	if (unblock) {
+		spin_lock(&dentry->d_lock);
+		store_release_wake_up(&dentry->d_flags,
+				      dentry->d_flags &~ DCACHE_BLOCKED);
+		spin_unlock(&dentry->d_lock);
+	}
 	return rc;
 }
 
@@ -2536,7 +2548,6 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 	struct cifs_sb_info *cifs_sb;
 	struct tcon_link *tlink;
 	struct cifs_tcon *tcon;
-	bool rehash = false;
 	unsigned int xid;
 	int rc, tmprc;
 	int retry_count = 0;
@@ -2552,20 +2563,20 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 	if (unlikely(cifs_forced_shutdown(cifs_sb)))
 		return smb_EIO(smb_eio_trace_forced_shutdown);
 
-	/*
-	 * Prevent any concurrent opens on the target by unhashing the dentry.
-	 * VFS already unhashes the target when renaming directories.
-	 */
-	if (d_is_positive(target_dentry) && !d_is_dir(target_dentry)) {
-		if (!d_unhashed(target_dentry)) {
-			d_drop(target_dentry);
-			rehash = true;
-		}
-	}
-
 	tlink = cifs_sb_tlink(cifs_sb);
 	if (IS_ERR(tlink))
 		return PTR_ERR(tlink);
+
+	/*
+	 * Block opens - and all lookups that involve d_revalidate.
+	 * This guarantees that if another thread tries to open(), it
+	 * will either block, or will increment d_count()
+	 * before we test it in __cifs_unlink().
+	 */
+	spin_lock(&target_dentry->d_lock);
+	target_dentry->d_flags |= DCACHE_BLOCKED;
+	spin_unlock(&target_dentry->d_lock);
+
 	tcon = tlink_tcon(tlink);
 	server = tcon->ses->server;
 
@@ -2605,8 +2616,6 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 		}
 	}
 
-	if (!rc)
-		rehash = false;
 	/*
 	 * No-replace is the natural behavior for CIFS, so skip unlink hacks.
 	 */
@@ -2698,8 +2707,6 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 			}
 			rc = cifs_do_rename(xid, source_dentry, from_name,
 					    target_dentry, to_name);
-			if (!rc)
-				rehash = false;
 		}
 	}
 
@@ -2713,8 +2720,11 @@ cifs_rename2(struct mnt_idmap *idmap, struct inode *source_dir,
 	CIFS_I(source_dir)->time = CIFS_I(target_dir)->time = 0;
 
 cifs_rename_exit:
-	if (rehash)
-		d_rehash(target_dentry);
+	/* Allow lookups/opens */
+	spin_lock(&target_dentry->d_lock);
+	store_release_wake_up(&target_dentry->d_flags,
+			      target_dentry->d_flags &~ DCACHE_BLOCKED);
+	spin_unlock(&target_dentry->d_lock);
 	kfree(info_buf_source);
 	free_dentry_path(page2);
 	free_dentry_path(page1);

base-commit: 3879f51857325da9bf3cfb073280257cd16ae067
-- 
2.50.0.107.gf914562f5916.dirty