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shrinking rcu_read_lock() scope in d_alloc_parallel()

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The current use of rcu_read_lock() uses in d_alloc_parallel()
is fairly opaque - the single large scope serves two purposes.

	We start with lookup in normal hash, and there rcu_read_lock()
scope puts __d_lookup_rcu() and subsequent lockref_get_not_dead() into
the same RCU read-side critical area.

	If no match is found, we proceed to lock the hash chain of
in-lookup hash and scan that for a match.  If we find a match, we want
to grab it and wait for lookup in progress to finish.  Since the bitlock
we use for these hash chains has to nest inside ->d_lock, we need to
unlock the chain first and use lockref_get_not_dead() on the match.
That has to be done without breaking the RCU read-side critical area,
and we use the same rcu_read_lock() scope to bridge over.

	The thing is, after having grabbed the reference (and it is
very unlikely to fail) we proceed to grab ->d_lock - d_wait_lookup()
and __d_lookup_unhash()/__d_wake_in_lookup_waiters() are using that for
serialization.	That makes lockref_get_not_dead() pointless - trying
to avoid grabbing ->d_lock for refcount increment, only to grab it
anyway immediately after that.	If we grab ->d_lock first and replace
lockref_get_not_dead() with direct check for sign and increment if
non-negative we can move rcu_read_unlock() to immediately after grabbing
->d_lock.  Moreover, we don't need the RCU read-side critical area to
be contiguous since before earlier __d_lookup_rcu() - we can just as
well terminate the earlier one ASAP and call rcu_read_lock() again only
after having found a match (if any) in the in-lookup hash chain.

	That makes the entire thing easier to follow and the purpose
of those rcu_read_lock() calls easier to describe - the first scope is
for __d_lookup_rcu() + lockref_get_not_dead(), the second one bridges
over from the bitlock scope to the ->d_lock scope on the match found in
in-lookup hash.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2026-04-23 19:29:18 +01:00
parent 3ef89feb38
commit 92cba84470
1 changed files with 12 additions and 17 deletions
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29
fs/dcache.c
View File

@ -2743,38 +2743,33 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
spin_unlock(&parent->d_lock);
retry:
rcu_read_lock();
seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
r_seq = read_seqbegin(&rename_lock);
rcu_read_lock();
dentry = __d_lookup_rcu(parent, name, &d_seq);
if (unlikely(dentry)) {
if (!lockref_get_not_dead(&dentry->d_lockref)) {
rcu_read_unlock();
goto retry;
}
rcu_read_unlock();
if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
rcu_read_unlock();
dput(dentry);
goto retry;
}
rcu_read_unlock();
dput(new);
return dentry;
}
if (unlikely(read_seqretry(&rename_lock, r_seq))) {
rcu_read_unlock();
rcu_read_unlock();
if (unlikely(read_seqretry(&rename_lock, r_seq)))
goto retry;
}
if (unlikely(seq & 1)) {
rcu_read_unlock();
if (unlikely(seq & 1))
goto retry;
}
hlist_bl_lock(b);
if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
hlist_bl_unlock(b);
rcu_read_unlock();
goto retry;
}
/*
@ -2791,19 +2786,20 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
continue;
if (!d_same_name(dentry, parent, name))
continue;
rcu_read_lock();
hlist_bl_unlock(b);
spin_lock(&dentry->d_lock);
rcu_read_unlock();
/* now we can try to grab a reference */
if (!lockref_get_not_dead(&dentry->d_lockref)) {
rcu_read_unlock();
if (unlikely(dentry->d_lockref.count < 0)) {
spin_unlock(&dentry->d_lock);
goto retry;
}
rcu_read_unlock();
/*
* somebody is likely to be still doing lookup for it;
* wait for them to finish
* pin it and wait for them to finish
*/
spin_lock(&dentry->d_lock);
dget_dlock(dentry);
d_wait_lookup(dentry);
/*
* it's not in-lookup anymore; in principle we should repeat
@ -2824,7 +2820,6 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
dput(new);
return dentry;
}
rcu_read_unlock();
hlist_bl_add_head(&new->d_in_lookup_hash, b);
hlist_bl_unlock(b);
return new;