Commit 84e710da authored by Al Viro's avatar Al Viro
Browse files

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: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 85c7f810

Documentation/filesystems/porting

+8 −0
Original line number Diff line number Diff line
@@ -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.

fs/dcache.c

+32 −2
Original line number Diff line number Diff line
@@ -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);

}

fs/inode.c

+1 −0
Original line number Diff line number Diff line
@@ -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;

include/linux/fs.h

+1 −0
Original line number Diff line number Diff line
@@ -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;

mm/shmem.c

+2 −1
Original line number Diff line number Diff line
@@ -3123,6 +3123,7 @@ 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);

	if (S_ISLNK(inode->i_mode))

		kfree(inode->i_link);

	kmem_cache_free(shmem_inode_cachep, SHMEM_I(inode));

}

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