[PATCH v3 03/20] make take_dentry_name_snapshot() lockless

Al Viro
2025-01-23T01:46:44Z

Use ->d_seq instead of grabbing ->d_lock; in case of shortname dentries
that avoids any stores to shared data objects and in case of long names
we are down to (unavoidable) atomic_inc on the external_name refcount.

Makes the thing safer as well - the areas where ->d_seq is held odd are
all nested inside the areas where ->d_lock is held, and the latter are
much more numerous.

NOTE: now that there is a lockless path where we might try to grab
a reference to an already doomed external_name instance, it is no
longer possible for external_name.u.count and external_name.u.head
to share space (kudos to Linus for spotting that).

To reduce the noise this commit just make external_name.u a struct
(instead of union); the next commit will dissolve it.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/dcache.c | 35 +++++++++++++++++++++++++----------
1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 52662a5d08e4..f387dc97df86 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -296,9 +296,9 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
}

struct external_name {
- union {
- atomic_t count;
- struct rcu_head head;
+ struct {
+ atomic_t count; // ->count and ->head can't be combined
+ struct rcu_head head; // see take_dentry_name_snapshot()
} u;
unsigned char name[];
};
@@ -329,15 +329,30 @@ static inline int dname_external(const struct dentry *dentry)

void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
{
- spin_lock(&dentry->d_lock);
- name->name = dentry->d_name;
- if (unlikely(dname_external(dentry))) {
- atomic_inc(&external_name(dentry)->u.count);
- } else {
+ unsigned seq;
+ const unsigned char *s;
+
+ rcu_read_lock();
+retry:
+ seq = read_seqcount_begin(&dentry->d_seq);
+ s = READ_ONCE(dentry->d_name.name);
+ name->name.hash_len = dentry->d_name.hash_len;
+ name->name.name = name->inline_name.string;
+ if (likely(s == dentry->d_shortname.string)) {
name->inline_name = dentry->d_shortname;
- name->name.name = name->inline_name.string;
+ } else {
+ struct external_name *p;
+ p = container_of(s, struct external_name, name[0]);
+ // get a valid reference
+ if (unlikely(!atomic_inc_not_zero(&p->u.count)))
+ goto retry;
+ name->name.name = s;
}
- spin_unlock(&dentry->d_lock);
+ if (read_seqcount_retry(&dentry->d_seq, seq)) {
+ release_dentry_name_snapshot(name);
+ goto retry;
+ }
+ rcu_read_unlock();
}
EXPORT_SYMBOL(take_dentry_name_snapshot);

--
2.39.5