[PATCH v2 04/19] VFS: use wait_var_event for waiting in d_alloc_parallel()

NeilBrown
2026-04-27T03:36:46Z

d_alloc_parallel() currently requires a wait_queue_head to be passed in.
This must have a life time which extends until the lookup is completed.

This makes it awkward to use and particularly make it hard to use in
lookup_one_qstr_excl() which I hope to do in the future.

This patch changes d_alloc_parallel() to use wake_up_var_locked() to
wake up waiters, and wait_var_event_spinlock() to wait. dentry->d_lock
is used for synchronisation as it is already held and the relevant
times.

In most cases there will be no waiters so the wake_up_var_locked()
call is a complete waste. To optimise this a new ->d_flags flag is
added: DCACHE_LOCK_WAITERS. This is set whenever any thread prepares to
wait for the dentry, and if it isn't set when DCACHE_PAR_LOOKUP is
cleared, no wakeup is sent.
(The name is deliberately generic as I plan to replace DCACHE_PAR_LOOKUP
with more generic per-dentry locking in the future).

__d_lookup_unhash() now returns a bool rather than a wq. This is true
if DCACHE_LOCK_WAITERS was sent and is used to decide to send the wake
up. It would be easier to send the wakeup immediately when clearing
DCACHE_LOCK_WAITERS, but then the waiter could wake a bit earlier and
then spend time spinning on ->d_lock. I don't know if that cost is
interesting.

__d_lookup_unhash() no longer needs to re-init ->d_lru. That was
previously shared (in a union) with ->d_wait but ->d_wait is now gone
so it no longer corrupts ->d_lru.

Signed-off-by: NeilBrown <neil@brown.name>
---
Documentation/filesystems/porting.rst | 7 +++
fs/afs/dir_silly.c | 4 +-
fs/dcache.c | 64 ++++++++++++---------------
fs/fuse/readdir.c | 3 +-
fs/namei.c | 6 +--
fs/nfs/dir.c | 6 +--
fs/nfs/unlink.c | 3 +-
fs/proc/base.c | 3 +-
fs/proc/proc_sysctl.c | 3 +-
fs/smb/client/readdir.c | 3 +-
include/linux/dcache.h | 11 +++--
include/linux/nfs_xdr.h | 1 -
12 files changed, 51 insertions(+), 63 deletions(-)

diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index bfdff4608028..85c7b2007f8c 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -1385,3 +1385,10 @@ for_each_alias(dentry, inode) instead of hlist_for_each_entry; better
yet, see if any of the exported primitives could be used instead of
the entire loop. You still need to hold ->i_lock of the inode over
either form of manual loop.
+
+---
+
+**mandatory**
+
+d_alloc_parallel() no longer requires a waitqueue_head.
+
diff --git a/fs/afs/dir_silly.c b/fs/afs/dir_silly.c
index a748fd133faf..982bb6ec15f0 100644
--- a/fs/afs/dir_silly.c
+++ b/fs/afs/dir_silly.c
@@ -248,13 +248,11 @@ int afs_silly_iput(struct dentry *dentry, struct inode *inode)
struct dentry *alias;
int ret;

- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
-
_enter("%p{%pd},%llx", dentry, dentry, vnode->fid.vnode);

down_read(&dvnode->rmdir_lock);

- alias = d_alloc_parallel(dentry->d_parent, &dentry->d_name, &wq);
+ alias = d_alloc_parallel(dentry->d_parent, &dentry->d_name);
if (IS_ERR(alias)) {
up_read(&dvnode->rmdir_lock);
return 0;
diff --git a/fs/dcache.c b/fs/dcache.c
index d0a504fc62e5..2dcefa60db32 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2268,8 +2268,7 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
return found;
}
if (d_in_lookup(dentry)) {
- found = d_alloc_parallel(dentry->d_parent, name,
- dentry->d_wait);
+ found = d_alloc_parallel(dentry->d_parent, name);
if (IS_ERR(found) || !d_in_lookup(found)) {
iput(inode);
return found;
@@ -2279,7 +2278,7 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
if (!found) {
iput(inode);
return ERR_PTR(-ENOMEM);
- }
+ }
}
res = d_splice_alias(inode, found);
if (res) {
@@ -2657,31 +2656,24 @@ static inline unsigned start_dir_add(struct inode *dir)
}

static inline void end_dir_add(struct inode *dir, unsigned int n,
- wait_queue_head_t *d_wait)
+ bool do_wake, struct dentry *de)
{
smp_store_release(&dir->i_dir_seq, n + 2);
preempt_enable_nested();
- if (wq_has_sleeper(d_wait))
- wake_up_all(d_wait);
+ if (do_wake)
+ wake_up_var_locked(&de->d_flags, &de->d_lock);
}

-static void d_wait_lookup(struct dentry *dentry)
+static inline bool d_must_wait(struct dentry *dentry)
{
- if (d_in_lookup(dentry)) {
- DECLARE_WAITQUEUE(wait, current);
- add_wait_queue(dentry->d_wait, &wait);
- do {
- set_current_state(TASK_UNINTERRUPTIBLE);
- spin_unlock(&dentry->d_lock);
- schedule();
- spin_lock(&dentry->d_lock);
- } while (d_in_lookup(dentry));
- }
+ if (!d_in_lookup(dentry))
+ return false;
+ dentry->d_flags |= DCACHE_LOCK_WAITER;
+ return true;
}

struct dentry *d_alloc_parallel(struct dentry *parent,
- const struct qstr *name,
- wait_queue_head_t *wq)
+ const struct qstr *name)
{
unsigned int hash = name->hash;
struct hlist_bl_head *b = in_lookup_hash(parent, hash);
@@ -2763,7 +2755,9 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
* wait for them to finish
*/
spin_lock(&dentry->d_lock);
- d_wait_lookup(dentry);
+ wait_var_event_spinlock(&dentry->d_flags,
+ !d_must_wait(dentry),
+ &dentry->d_lock);
/*
* it's not in-lookup anymore; in principle we should repeat
* everything from dcache lookup, but it's likely to be what
@@ -2784,7 +2778,6 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
return dentry;
}
rcu_read_unlock();
- new->d_wait = wq;
hlist_bl_add_head(&new->d_in_lookup_hash, b);
hlist_bl_unlock(b);
return new;
@@ -2800,29 +2793,29 @@ EXPORT_SYMBOL(d_alloc_parallel);
* - Retrieve and clear the waitqueue head in dentry
* - Return the waitqueue head
*/
-static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
+static bool __d_lookup_unhash(struct dentry *dentry)
{
- wait_queue_head_t *d_wait;
struct hlist_bl_head *b;
+ bool do_wake;

lockdep_assert_held(&dentry->d_lock);

b = in_lookup_hash(dentry->d_parent, dentry->d_name.hash);
hlist_bl_lock(b);
- dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
__hlist_bl_del(&dentry->d_in_lookup_hash);
- d_wait = dentry->d_wait;
- dentry->d_wait = NULL;
+ do_wake = !!(dentry->d_flags & DCACHE_LOCK_WAITER);
+ dentry->d_flags &= ~(DCACHE_PAR_LOOKUP|DCACHE_LOCK_WAITER);
+
hlist_bl_unlock(b);
dentry->waiters = NULL;
- INIT_LIST_HEAD(&dentry->d_lru);
- return d_wait;
+ return do_wake;
}

void __d_lookup_unhash_wake(struct dentry *dentry)
{
spin_lock(&dentry->d_lock);
- wake_up_all(__d_lookup_unhash(dentry));
+ if (__d_lookup_unhash(dentry))
+ wake_up_var_locked(&dentry->d_flags, &dentry->d_lock);
spin_unlock(&dentry->d_lock);
}
EXPORT_SYMBOL(__d_lookup_unhash_wake);
@@ -2832,14 +2825,15 @@ EXPORT_SYMBOL(__d_lookup_unhash_wake);
static inline void __d_add(struct dentry *dentry, struct inode *inode,
const struct dentry_operations *ops)
{
- wait_queue_head_t *d_wait;
+ bool do_wake = false;
struct inode *dir = NULL;
unsigned n;
+
spin_lock(&dentry->d_lock);
if (unlikely(d_in_lookup(dentry))) {
dir = dentry->d_parent->d_inode;
n = start_dir_add(dir);
- d_wait = __d_lookup_unhash(dentry);
+ do_wake = __d_lookup_unhash(dentry);
__d_rehash(dentry);
} else if (d_unhashed(dentry)) {
__d_rehash(dentry);
@@ -2849,7 +2843,7 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode,
if (inode)
__d_instantiate(dentry, inode);
if (dir)
- end_dir_add(dir, n, d_wait);
+ end_dir_add(dir, n, do_wake, dentry);
spin_unlock(&dentry->d_lock);
if (inode)
spin_unlock(&inode->i_lock);
@@ -2962,7 +2956,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
bool exchange)
{
struct dentry *old_parent, *p;
- wait_queue_head_t *d_wait;
+ bool do_wake = false;
struct inode *dir = NULL;
unsigned n;

@@ -2993,7 +2987,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
if (unlikely(d_in_lookup(target))) {
dir = target->d_parent->d_inode;
n = start_dir_add(dir);
- d_wait = __d_lookup_unhash(target);
+ do_wake = __d_lookup_unhash(target);
}

write_seqcount_begin(&dentry->d_seq);
@@ -3033,7 +3027,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
write_seqcount_end(&dentry->d_seq);

if (dir)
- end_dir_add(dir, n, d_wait);
+ end_dir_add(dir, n, do_wake, target);

if (dentry->d_parent != old_parent)
spin_unlock(&dentry->d_parent->d_lock);
diff --git a/fs/fuse/readdir.c b/fs/fuse/readdir.c
index db5ae8ec1030..a2361f1d9905 100644
--- a/fs/fuse/readdir.c
+++ b/fs/fuse/readdir.c
@@ -164,7 +164,6 @@ static int fuse_direntplus_link(struct file *file,
struct inode *dir = d_inode(parent);
struct fuse_conn *fc;
struct inode *inode;
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
int epoch;

if (!o->nodeid) {
@@ -201,7 +200,7 @@ static int fuse_direntplus_link(struct file *file,
dentry = d_lookup(parent, &name);
if (!dentry) {
retry:
- dentry = d_alloc_parallel(parent, &name, &wq);
+ dentry = d_alloc_parallel(parent, &name);
if (IS_ERR(dentry))
return PTR_ERR(dentry);
}
diff --git a/fs/namei.c b/fs/namei.c
index 65e60536a6d1..a6349b31fdb6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1891,13 +1891,12 @@ static struct dentry *__lookup_slow(const struct qstr *name,
{
struct dentry *dentry, *old;
struct inode *inode = dir->d_inode;
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);

/* Don't go there if it's already dead */
if (unlikely(IS_DEADDIR(inode)))
return ERR_PTR(-ENOENT);
again:
- dentry = d_alloc_parallel(dir, name, &wq);
+ dentry = d_alloc_parallel(dir, name);
if (IS_ERR(dentry))
return dentry;
if (unlikely(!d_in_lookup(dentry))) {
@@ -4414,7 +4413,6 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
struct dentry *dentry;
int error, create_error = 0;
umode_t mode = op->mode;
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);

if (unlikely(IS_DEADDIR(dir_inode)))
return ERR_PTR(-ENOENT);
@@ -4423,7 +4421,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
dentry = d_lookup(dir, &nd->last);
for (;;) {
if (!dentry) {
- dentry = d_alloc_parallel(dir, &nd->last, &wq);
+ dentry = d_alloc_parallel(dir, &nd->last);
if (IS_ERR(dentry))
return dentry;
}
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index e9ce1883288c..9580af999d70 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -726,7 +726,6 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry,
unsigned long dir_verifier)
{
struct qstr filename = QSTR_INIT(entry->name, entry->len);
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
struct dentry *dentry;
struct dentry *alias;
struct inode *inode;
@@ -755,7 +754,7 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry,
dentry = d_lookup(parent, &filename);
again:
if (!dentry) {
- dentry = d_alloc_parallel(parent, &filename, &wq);
+ dentry = d_alloc_parallel(parent, &filename);
if (IS_ERR(dentry))
return;
}
@@ -2106,7 +2105,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
struct file *file, unsigned open_flags,
umode_t mode)
{
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
struct nfs_open_context *ctx;
struct dentry *res;
struct iattr attr = { .ia_valid = ATTR_OPEN };
@@ -2162,7 +2160,7 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
d_drop(dentry);
switched = true;
dentry = d_alloc_parallel(dentry->d_parent,
- &dentry->d_name, &wq);
+ &dentry->d_name);
if (IS_ERR(dentry))
return PTR_ERR(dentry);
if (unlikely(!d_in_lookup(dentry)))
diff --git a/fs/nfs/unlink.c b/fs/nfs/unlink.c
index df3ca4669df6..43ea897943c0 100644
--- a/fs/nfs/unlink.c
+++ b/fs/nfs/unlink.c
@@ -124,7 +124,7 @@ static int nfs_call_unlink(struct dentry *dentry, struct inode *inode, struct nf
struct dentry *alias;

down_read_non_owner(&NFS_I(dir)->rmdir_sem);
- alias = d_alloc_parallel(dentry->d_parent, &data->args.name, &data->wq);
+ alias = d_alloc_parallel(dentry->d_parent, &data->args.name);
if (IS_ERR(alias)) {
up_read_non_owner(&NFS_I(dir)->rmdir_sem);
return 0;
@@ -185,7 +185,6 @@ nfs_async_unlink(struct dentry *dentry, const struct qstr *name)

data->cred = get_current_cred();
data->res.dir_attr = &data->dir_attr;
- init_waitqueue_head(&data->wq);

status = -EBUSY;
spin_lock(&dentry->d_lock);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index d9acfa89c894..d55a4b603188 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2132,8 +2132,7 @@ bool proc_fill_cache(struct file *file, struct dir_context *ctx,
goto end_instantiate;

if (!child) {
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
- child = d_alloc_parallel(dir, &qname, &wq);
+ child = d_alloc_parallel(dir, &qname);
if (IS_ERR(child))
goto end_instantiate;
if (d_in_lookup(child)) {
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 49ab74e0bfde..04a382178c65 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -692,8 +692,7 @@ static bool proc_sys_fill_cache(struct file *file,

child = d_lookup(dir, &qname);
if (!child) {
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
- child = d_alloc_parallel(dir, &qname, &wq);
+ child = d_alloc_parallel(dir, &qname);
if (IS_ERR(child))
return false;
if (d_in_lookup(child)) {
diff --git a/fs/smb/client/readdir.c b/fs/smb/client/readdir.c
index be22bbc4a65a..a8995261831c 100644
--- a/fs/smb/client/readdir.c
+++ b/fs/smb/client/readdir.c
@@ -73,7 +73,6 @@ cifs_prime_dcache(struct dentry *parent, struct qstr *name,
struct cifs_sb_info *cifs_sb = CIFS_SB(sb);
bool posix = cifs_sb_master_tcon(cifs_sb)->posix_extensions;
bool reparse_need_reval = false;
- DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
int rc;

cifs_dbg(FYI, "%s: for %s\n", __func__, name->name);
@@ -105,7 +104,7 @@ cifs_prime_dcache(struct dentry *parent, struct qstr *name,
(fattr->cf_flags & CIFS_FATTR_NEED_REVAL))
return;

- dentry = d_alloc_parallel(parent, name, &wq);
+ dentry = d_alloc_parallel(parent, name);
}
if (IS_ERR(dentry))
return;
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 2577c05f84ec..14b91a7d0bb6 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -116,10 +116,7 @@ struct dentry {
* possible!
*/

- union {
- struct list_head d_lru; /* LRU list */
- wait_queue_head_t *d_wait; /* in-lookup ones only */
- };
+ struct list_head d_lru; /* LRU list */
struct hlist_node d_sib; /* child of parent list */
struct hlist_head d_children; /* our children */
/*
@@ -210,6 +207,9 @@ enum dentry_flags {
DCACHE_REFERENCED = BIT(6), /* Recently used, don't discard. */
DCACHE_DONTCACHE = BIT(7), /* Purge from memory on final dput() */
DCACHE_CANT_MOUNT = BIT(8),
+ DCACHE_LOCK_WAITER = BIT(9), /* A thread is waiting for
+ * PAR_LOOKUP to clear
+ */
DCACHE_SHRINK_LIST = BIT(10),
DCACHE_OP_WEAK_REVALIDATE = BIT(11),
/*
@@ -256,8 +256,7 @@ extern void d_delete(struct dentry *);
/* allocate/de-allocate */
extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
extern struct dentry * d_alloc_anon(struct super_block *);
-extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *,
- wait_queue_head_t *);
+extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *);
extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
/* weird procfs mess; *NOT* exported */
extern struct dentry * d_splice_alias_ops(struct inode *, struct dentry *,
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index fcbd21b5685f..6aced49d5f00 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -1743,7 +1743,6 @@ struct nfs_unlinkdata {
struct nfs_removeargs args;
struct nfs_removeres res;
struct dentry *dentry;
- wait_queue_head_t wq;
const struct cred *cred;
struct nfs_fattr dir_attr;
long timeout;
--
2.50.0.107.gf914562f5916.dirty