提交 84e710da 编写于 作者: A Al Viro

parallel lookups machinery, part 2

We'll need to verify that there's neither a hashed nor in-lookup
dentry with desired parent/name before adding to in-lookup set.

One possible solution would be to hold the parent's ->d_lock through
both checks, but while the in-lookup set is relatively small at any
time, dcache is not.  And holding the parent's ->d_lock through
something like __d_lookup_rcu() would suck too badly.

So we leave the parent's ->d_lock alone, which means that we watch
out for the following scenario:
	* we verify that there's no hashed match
	* existing in-lookup match gets hashed by another process
	* we verify that there's no in-lookup matches and decide
that everything's fine.

Solution: per-directory kinda-sorta seqlock, bumped around the times
we hash something that used to be in-lookup or move (and hash)
something in place of in-lookup.  Then the above would turn into
	* read the counter
	* do dcache lookup
	* if no matches found, check for in-lookup matches
	* if there had been none of those either, check if the
counter has changed; repeat if it has.

The "kinda-sorta" part is due to the fact that we don't have much spare
space in inode.  There is a spare word (shared with i_bdev/i_cdev/i_pipe),
so the counter part is not a problem, but spinlock is a different story.

We could use the parent's ->d_lock, and it would be less painful in
terms of contention, for __d_add() it would be rather inconvenient to
grab; we could do that (using lock_parent()), but...

Fortunately, we can get serialization on the counter itself, and it
might be a good idea in general; we can use cmpxchg() in a loop to
get from even to odd and smp_store_release() from odd to even.

This commit adds the counter and updating logics; the readers will be
added in the next commit.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
上级 85c7f810
......@@ -531,3 +531,11 @@ in your dentry operations instead.
dentry might be yet to be attached to inode, so do _not_ use its ->d_inode
in the instances. Rationale: !@#!@# security_d_instantiate() needs to be
called before we attach dentry to inode.
--
[mandatory]
symlinks are no longer the only inodes that do *not* have i_bdev/i_cdev/
i_pipe/i_link union zeroed out at inode eviction. As the result, you can't
assume that non-NULL value in ->i_nlink at ->destroy_inode() implies that
it's a symlink. Checking ->i_mode is really needed now. In-tree we had
to fix shmem_destroy_callback() that used to take that kind of shortcut;
watch out, since that shortcut is no longer valid.
......@@ -2364,6 +2364,22 @@ void d_rehash(struct dentry * entry)
}
EXPORT_SYMBOL(d_rehash);
static inline unsigned start_dir_add(struct inode *dir)
{
for (;;) {
unsigned n = dir->i_dir_seq;
if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
return n;
cpu_relax();
}
}
static inline void end_dir_add(struct inode *dir, unsigned n)
{
smp_store_release(&dir->i_dir_seq, n + 2);
}
void __d_lookup_done(struct dentry *dentry)
{
dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
......@@ -2375,9 +2391,14 @@ EXPORT_SYMBOL(__d_lookup_done);
static inline void __d_add(struct dentry *dentry, struct inode *inode)
{
struct inode *dir = NULL;
unsigned n;
spin_lock(&dentry->d_lock);
if (unlikely(d_in_lookup(dentry)))
if (unlikely(d_in_lookup(dentry))) {
dir = dentry->d_parent->d_inode;
n = start_dir_add(dir);
__d_lookup_done(dentry);
}
if (inode) {
unsigned add_flags = d_flags_for_inode(inode);
hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
......@@ -2387,6 +2408,8 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
__fsnotify_d_instantiate(dentry);
}
_d_rehash(dentry);
if (dir)
end_dir_add(dir, n);
spin_unlock(&dentry->d_lock);
if (inode)
spin_unlock(&inode->i_lock);
......@@ -2616,6 +2639,8 @@ static void dentry_unlock_for_move(struct dentry *dentry, struct dentry *target)
static void __d_move(struct dentry *dentry, struct dentry *target,
bool exchange)
{
struct inode *dir = NULL;
unsigned n;
if (!dentry->d_inode)
printk(KERN_WARNING "VFS: moving negative dcache entry\n");
......@@ -2623,8 +2648,11 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
BUG_ON(d_ancestor(target, dentry));
dentry_lock_for_move(dentry, target);
if (unlikely(d_in_lookup(target)))
if (unlikely(d_in_lookup(target))) {
dir = target->d_parent->d_inode;
n = start_dir_add(dir);
__d_lookup_done(target);
}
write_seqcount_begin(&dentry->d_seq);
write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
......@@ -2674,6 +2702,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
write_seqcount_end(&target->d_seq);
write_seqcount_end(&dentry->d_seq);
if (dir)
end_dir_add(dir, n);
dentry_unlock_for_move(dentry, target);
}
......
......@@ -151,6 +151,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
inode->i_bdev = NULL;
inode->i_cdev = NULL;
inode->i_link = NULL;
inode->i_dir_seq = 0;
inode->i_rdev = 0;
inode->dirtied_when = 0;
......
......@@ -684,6 +684,7 @@ struct inode {
struct block_device *i_bdev;
struct cdev *i_cdev;
char *i_link;
unsigned i_dir_seq;
};
__u32 i_generation;
......
......@@ -3123,7 +3123,8 @@ static struct inode *shmem_alloc_inode(struct super_block *sb)
static void shmem_destroy_callback(struct rcu_head *head)
{
struct inode *inode = container_of(head, struct inode, i_rcu);
kfree(inode->i_link);
if (S_ISLNK(inode->i_mode))
kfree(inode->i_link);
kmem_cache_free(shmem_inode_cachep, SHMEM_I(inode));
}
......
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