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make take_dentry_name_snapshot() lockless

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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>
This commit is contained in:
Al Viro 2024-12-08 22:27:51 -05:00
parent 58cf9c383c
commit 1c9be84a90
1 changed files with 25 additions and 10 deletions
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35
fs/dcache.c
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@ -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);