[PATCH] dcache: remove unnecessary NULL check in dget_dlock()

[PATCH] dcache: remove unnecessary NULL check in dget_dlock()

[Vegard Nossum]

dget_dlock() requires dentry->d_lock to be held when called, yet
contains a NULL check for dentry.

An audit of all calls to dget_dlock() shows that it is never called
with a NULL pointer (as spin_lock()/spin_unlock() would crash in these
cases):

  $ git grep -W '\<dget_dlock\>'

  arch/powerpc/platforms/cell/spufs/inode.c-              spin_lock(&dentry->d_lock);
  arch/powerpc/platforms/cell/spufs/inode.c-              if (simple_positive(dentry)) {
  arch/powerpc/platforms/cell/spufs/inode.c:                      dget_dlock(dentry);

  fs/autofs/expire.c-             spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
  fs/autofs/expire.c-             if (simple_positive(child)) {
  fs/autofs/expire.c:                     dget_dlock(child);

  fs/autofs/root.c:                       dget_dlock(active);
  fs/autofs/root.c-                       spin_unlock(&active->d_lock);

  fs/autofs/root.c:                       dget_dlock(expiring);
  fs/autofs/root.c-                       spin_unlock(&expiring->d_lock);

  fs/ceph/dir.c-          if (!spin_trylock(&dentry->d_lock))
  fs/ceph/dir.c-                  continue;
  [...]
  fs/ceph/dir.c:                          dget_dlock(dentry);

  fs/ceph/mds_client.c-           spin_lock(&alias->d_lock);
  [...]
  fs/ceph/mds_client.c:                   dn = dget_dlock(alias);

  fs/configfs/inode.c-            spin_lock(&dentry->d_lock);
  fs/configfs/inode.c-            if (simple_positive(dentry)) {
  fs/configfs/inode.c:                    dget_dlock(dentry);

  fs/libfs.c:                             found = dget_dlock(d);
  fs/libfs.c-                     spin_unlock(&d->d_lock);

  fs/libfs.c:             found = dget_dlock(child);
  fs/libfs.c-     spin_unlock(&child->d_lock);

  fs/libfs.c:                             child = dget_dlock(d);
  fs/libfs.c-                     spin_unlock(&d->d_lock);

  fs/ocfs2/dcache.c:                      dget_dlock(dentry);
  fs/ocfs2/dcache.c-                      spin_unlock(&dentry->d_lock);

  include/linux/dcache.h:static inline struct dentry *dget_dlock(struct dentry *dentry)

After taking out the NULL check, dget_dlock() becomes almost identical
to __dget_dlock(); the only difference is that dget_dlock() returns the
dentry that was passed in. These are static inline helpers, so we can
rely on the compiler to discard unused return values. We can therefore
also remove __dget_dlock() and replace calls to it by dget_dlock().

Also fix up and improve the kerneldoc comments while we're at it.

Testing: x86 defconfig build + boot; make htmldocs for the kerneldoc
warning. objdump shows there are code generation changes.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 fs/dcache.c            | 14 ++++----------
 include/linux/dcache.h | 20 ++++++++++++++------
 2 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 25ac74d30bff..8c3c0d691605 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -942,12 +942,6 @@ void dput_to_list(struct dentry *dentry, struct list_head *list)
 	spin_unlock(&dentry->d_lock);
 }
 
-/* This must be called with d_lock held */
-static inline void __dget_dlock(struct dentry *dentry)
-{
-	dentry->d_lockref.count++;
-}
-
 static inline void __dget(struct dentry *dentry)
 {
 	lockref_get(&dentry->d_lockref);
@@ -1034,7 +1028,7 @@ static struct dentry *__d_find_alias(struct inode *inode)
 	hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
 		spin_lock(&alias->d_lock);
  		if (!d_unhashed(alias)) {
-			__dget_dlock(alias);
+			dget_dlock(alias);
 			spin_unlock(&alias->d_lock);
 			return alias;
 		}
@@ -1707,7 +1701,7 @@ static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
 {
 	struct dentry **victim = _data;
 	if (d_mountpoint(dentry)) {
-		__dget_dlock(dentry);
+		dget_dlock(dentry);
 		*victim = dentry;
 		return D_WALK_QUIT;
 	}
@@ -1853,7 +1847,7 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 	 * don't need child lock because it is not subject
 	 * to concurrency here
 	 */
-	__dget_dlock(parent);
+	dget_dlock(parent);
 	dentry->d_parent = parent;
 	list_add(&dentry->d_child, &parent->d_subdirs);
 	spin_unlock(&parent->d_lock);
@@ -2851,7 +2845,7 @@ struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
 			spin_unlock(&alias->d_lock);
 			alias = NULL;
 		} else {
-			__dget_dlock(alias);
+			dget_dlock(alias);
 			__d_rehash(alias);
 			spin_unlock(&alias->d_lock);
 		}
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 6b351e009f59..e22f1520ea3b 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -300,20 +300,28 @@ extern char *dentry_path(const struct dentry *, char *, int);
 /* Allocation counts.. */
 
 /**
- *	dget, dget_dlock -	get a reference to a dentry
+ *	dget_dlock -	get a reference to a dentry
  *	@dentry: dentry to get a reference to
  *
- *	Given a dentry or %NULL pointer increment the reference count
- *	if appropriate and return the dentry. A dentry will not be 
- *	destroyed when it has references.
+ *	Given a dentry, increment the reference count and return the
+ *	dentry.
+ *
+ *	Context: @dentry->d_lock must be held.
  */
 static inline struct dentry *dget_dlock(struct dentry *dentry)
 {
-	if (dentry)
-		dentry->d_lockref.count++;
+	dentry->d_lockref.count++;
 	return dentry;
 }
 
+/**
+ *	dget -	get a reference to a dentry
+ *	@dentry: dentry to get a reference to
+ *
+ *	Given a dentry or %NULL pointer increment the reference count
+ *	if appropriate and return the dentry. A dentry will not be
+ *	destroyed when it has references.
+ */
 static inline struct dentry *dget(struct dentry *dentry)
 {
 	if (dentry)
-- 
2.34.1

Re: [PATCH] dcache: remove unnecessary NULL check in dget_dlock()

[Al Viro]

On Sun, Oct 22, 2023 at 06:45:20PM +0200, Vegard Nossum wrote:

> @@ -1707,7 +1701,7 @@ static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
>  {
>  	struct dentry **victim = _data;
>  	if (d_mountpoint(dentry)) {
> -		__dget_dlock(dentry);
> +		dget_dlock(dentry);
>  		*victim = dentry;

		*victim = dget_dlock(dentry);

>  		return D_WALK_QUIT;
>  	}
> @@ -1853,7 +1847,7 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
>  	 * don't need child lock because it is not subject
>  	 * to concurrency here
>  	 */
> -	__dget_dlock(parent);
> +	dget_dlock(parent);
>  	dentry->d_parent = parent;

	dentry->d_parent = dget_dlock(parent);

> - *	Given a dentry or %NULL pointer increment the reference count
> - *	if appropriate and return the dentry. A dentry will not be 
> - *	destroyed when it has references.
> + *	Given a dentry, increment the reference count and return the
> + *	dentry.
> + *
> + *	Context: @dentry->d_lock must be held.

... and dentry must be alive.  There are many ways the caller could
use to guarantee that - any of the "it's hashed", "it's positive",
"its ->d_lockref.count is not negative" would suffice under ->d_lock.