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SubjectRe: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())
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On 2018-03-14, Al Viro <viro@ZenIV.linux.org.uk> wrote:
>>> + rcu_read_lock(); /* to protect parent */
>>> + spin_unlock(&dentry->d_lock);
>>> + parent = READ_ONCE(dentry->d_parent);
>>
>> The preceeding line should be removed. We already have a "parent"
>> from before we did the most recent trylock().
>
> Nope. We have parent, yes, but it had been fetched outside of
> rcu_read_lock(). So the object it used to point to might have been
> already freed and we can't do this:
>
>>> + spin_lock(&parent->d_lock);

When rcu_read_lock() is called, we are still holding dentry->d_lock. At
that point dentry->d_parent cannot have changed and cannot have been
freed. So the parent fetched outside of rcu_read_lock() is also
protected from freeing inside that rcu_read_lock().

> Come to think of that, it might make sense to lift rcu_read_lock() all
> the way out of that sucker.

Agreed.

> Objections? Below is the incremental I'd fold into that commit:
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index f0e73c93182b..0d1dac750c0a 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1000,7 +1000,6 @@ static bool shrink_lock_dentry(struct dentry *dentry)
>
> inode = dentry->d_inode;
> if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
> - rcu_read_lock(); /* to protect inode */
> spin_unlock(&dentry->d_lock);
> spin_lock(&inode->i_lock);
> spin_lock(&dentry->d_lock);
> @@ -1009,16 +1008,14 @@ static bool shrink_lock_dentry(struct dentry *dentry)
> /* changed inode means that somebody had grabbed it */
> if (unlikely(inode != dentry->d_inode))
> goto out;
> - rcu_read_unlock();
> }
>
> parent = dentry->d_parent;
> + /* parent will stay allocated until we drop rcu_read_lock */

I think this comment is not necessary since this function no longer
deals with dropping rcu_read_lock. But if we keep it, it should be added
for the inode above as well.

> if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
> return true;
>
> - rcu_read_lock(); /* to protect parent */
> spin_unlock(&dentry->d_lock);
> - parent = READ_ONCE(dentry->d_parent);
> spin_lock(&parent->d_lock);
> if (unlikely(parent != dentry->d_parent)) {
> spin_unlock(&parent->d_lock);
> @@ -1026,14 +1023,11 @@ static bool shrink_lock_dentry(struct dentry *dentry)
> goto out;
> }
> spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> - if (likely(!dentry->d_lockref.count)) {
> - rcu_read_unlock();
> + if (likely(!dentry->d_lockref.count))
> return true;
> - }
> spin_unlock(&parent->d_lock);
> out:
> spin_unlock(&inode->i_lock);
> - rcu_read_unlock();
> return false;
> }
>
> @@ -1044,8 +1038,10 @@ static void shrink_dentry_list(struct list_head *list)
>
> dentry = list_entry(list->prev, struct dentry, d_lru);
> spin_lock(&dentry->d_lock);
> + rcu_read_lock();
> if (!shrink_lock_dentry(dentry)) {
> bool can_free = false;
> + rcu_read_unlock();
> d_shrink_del(dentry);
> if (dentry->d_lockref.count < 0)
> can_free = dentry->d_flags & DCACHE_MAY_FREE;
> @@ -1054,6 +1050,7 @@ static void shrink_dentry_list(struct list_head *list)
> dentry_free(dentry);
> continue;
> }
> + rcu_read_unlock();
> d_shrink_del(dentry);
> parent = dentry->d_parent;
> __dentry_kill(dentry);

John Ogness

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