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, 6 insertions, 4 deletions

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index a3e22803cddf..5635453cd476 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -254,8 +254,11 @@ struct dentry *proc_lookup_de(struct inode *dir, struct dentry *dentry,
 		inode = proc_get_inode(dir->i_sb, de);
 		if (!inode)
 			return ERR_PTR(-ENOMEM);
-		d_set_d_op(dentry, de->proc_dops);
-		return d_splice_alias(inode, dentry);
+		if (de->flags & PROC_ENTRY_FORCE_LOOKUP)
+			return d_splice_alias_ops(inode, dentry,
+						  &proc_net_dentry_ops);
+		return d_splice_alias_ops(inode, dentry,
+					  &proc_misc_dentry_ops);
 	}
 	read_unlock(&proc_subdir_lock);
 	return ERR_PTR(-ENOENT);
@@ -448,9 +451,8 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
 	INIT_LIST_HEAD(&ent->pde_openers);
 	proc_set_user(ent, (*parent)->uid, (*parent)->gid);
 
-	ent->proc_dops = &proc_misc_dentry_ops;
 	/* Revalidate everything under /proc/${pid}/net */
-	if ((*parent)->proc_dops == &proc_net_dentry_ops)
+	if ((*parent)->flags & PROC_ENTRY_FORCE_LOOKUP)
 		pde_force_lookup(ent);
 
 out: