Merge tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull dentry d_flags updates from Al Viro:
 "The current exclusion rules for dentry->d_flags stores are rather
  unpleasant. The basic rules are simple:

   - stores to dentry->d_flags are OK under dentry->d_lock

   - stores to dentry->d_flags are OK in the dentry constructor, before
     becomes potentially visible to other threads

  Unfortunately, there's a couple of exceptions to that, and that's
  where the headache comes from.

  The main PITA comes from d_set_d_op(); that primitive sets ->d_op of
  dentry and adjusts the flags that correspond to presence of individual
  methods. It's very easy to misuse; existing uses _are_ safe, but proof
  of correctness is brittle.

  Use in __d_alloc() is safe (we are within a constructor), but we might
  as well precalculate the initial value of 'd_flags' when we set the
  default ->d_op for given superblock and set 'd_flags' directly instead
  of messing with that helper.

  The reasons why other uses are safe are bloody convoluted; I'm not
  going to reproduce it here. See [1] for gory details, if you care. The
  critical part is using d_set_d_op() only just prior to
  d_splice_alias(), which makes a combination of d_splice_alias() with
  setting ->d_op, etc a natural replacement primitive.

  Better yet, if we go that way, it's easy to take setting ->d_op and
  modifying 'd_flags' under ->d_lock, which eliminates the headache as
  far as 'd_flags' exclusion rules are concerned. Other exceptions are
  minor and easy to deal with.

  What this series does:

   - d_set_d_op() is no longer available; instead a new primitive
     (d_splice_alias_ops()) is provided, equivalent to combination of
     d_set_d_op() and d_splice_alias().

   - new field of struct super_block - 's_d_flags'. This sets the
     default value of 'd_flags' to be used when allocating dentries on
     this filesystem.

   - new primitive for setting 's_d_op': set_default_d_op(). This
     replaces stores to 's_d_op' at mount time.

     All in-tree filesystems converted; out-of-tree ones will get caught
     by the compiler ('s_d_op' is renamed, so stores to it will be
     caught). 's_d_flags' is set by the same primitive to match the
     's_d_op'.

   - a lot of filesystems had sb->s_d_op->d_delete equal to
     always_delete_dentry; that is equivalent to setting
     DCACHE_DONTCACHE in 'd_flags', so such filesystems can bloody well
     set that bit in 's_d_flags' and drop 'd_delete()' from
     dentry_operations.

     In quite a few cases that results in empty dentry_operations, which
     means that we can get rid of those.

   - kill simple_dentry_operations - not needed anymore

   - massage d_alloc_parallel() to get rid of the other exception wrt
     'd_flags' stores - we can set DCACHE_PAR_LOOKUP as soon as we
     allocate the new dentry; no need to delay that until we commit to
     using the sucker.

  As the result, 'd_flags' stores are all either under ->d_lock or done
  before the dentry becomes visible in any shared data structures"

Link: https://lore.kernel.org/all/20250224010624.GT1977892@ZenIV/ [1]

* tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (21 commits)
  configfs: use DCACHE_DONTCACHE
  debugfs: use DCACHE_DONTCACHE
  efivarfs: use DCACHE_DONTCACHE instead of always_delete_dentry()
  9p: don't bother with always_delete_dentry
  ramfs, hugetlbfs, mqueue: set DCACHE_DONTCACHE
  kill simple_dentry_operations
  devpts, sunrpc, hostfs: don't bother with ->d_op
  shmem: no dentry retention past the refcount reaching zero
  d_alloc_parallel(): set DCACHE_PAR_LOOKUP earlier
  make d_set_d_op() static
  simple_lookup(): just set DCACHE_DONTCACHE
  tracefs: Add d_delete to remove negative dentries
  set_default_d_op(): calculate the matching value for ->d_flags
  correct the set of flags forbidden at d_set_d_op() time
  split d_flags calculation out of d_set_d_op()
  new helper: set_default_d_op()
  fuse: no need for special dentry_operations for root dentry
  switch procfs from d_set_d_op() to d_splice_alias_ops()
  new helper: d_splice_alias_ops()
  procfs: kill ->proc_dops
  ...

1 files changed, 12 insertions, 15 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index 6f487fc6be34..54141236b881 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -62,11 +62,6 @@ int always_delete_dentry(const struct dentry *dentry)
 }
 EXPORT_SYMBOL(always_delete_dentry);
 
-const struct dentry_operations simple_dentry_operations = {
-	.d_delete = always_delete_dentry,
-};
-EXPORT_SYMBOL(simple_dentry_operations);
-
 /*
  * Lookup the data. This is trivial - if the dentry didn't already
  * exist, we know it is negative.  Set d_op to delete negative dentries.
@@ -75,9 +70,11 @@ struct dentry *simple_lookup(struct inode *dir, struct dentry *dentry, unsigned
 {
 	if (dentry->d_name.len > NAME_MAX)
 		return ERR_PTR(-ENAMETOOLONG);
-	if (!dentry->d_sb->s_d_op)
-		d_set_d_op(dentry, &simple_dentry_operations);
-
+	if (!dentry->d_op && !(dentry->d_flags & DCACHE_DONTCACHE)) {
+		spin_lock(&dentry->d_lock);
+		dentry->d_flags |= DCACHE_DONTCACHE;
+		spin_unlock(&dentry->d_lock);
+	}
 	if (IS_ENABLED(CONFIG_UNICODE) && IS_CASEFOLDED(dir))
 		return NULL;
 
@@ -684,7 +681,7 @@ static int pseudo_fs_fill_super(struct super_block *s, struct fs_context *fc)
 	s->s_root = d_make_root(root);
 	if (!s->s_root)
 		return -ENOMEM;
-	s->s_d_op = ctx->dops;
+	set_default_d_op(s, ctx->dops);
 	return 0;
 }
 
@@ -1948,22 +1945,22 @@ static const struct dentry_operations generic_encrypted_dentry_ops = {
  * @sb: superblock to be configured
  *
  * Filesystems supporting casefolding and/or fscrypt can call this
- * helper at mount-time to configure sb->s_d_op to best set of dentry
- * operations required for the enabled features. The helper must be
- * called after these have been configured, but before the root dentry
- * is created.
+ * helper at mount-time to configure default dentry_operations to the
+ * best set of dentry operations required for the enabled features.
+ * The helper must be called after these have been configured, but
+ * before the root dentry is created.
  */
 void generic_set_sb_d_ops(struct super_block *sb)
 {
 #if IS_ENABLED(CONFIG_UNICODE)
 	if (sb->s_encoding) {
-		sb->s_d_op = &generic_ci_dentry_ops;
+		set_default_d_op(sb, &generic_ci_dentry_ops);
 		return;
 	}
 #endif
 #ifdef CONFIG_FS_ENCRYPTION
 	if (sb->s_cop) {
-		sb->s_d_op = &generic_encrypted_dentry_ops;
+		set_default_d_op(sb, &generic_encrypted_dentry_ops);
 		return;
 	}
 #endif