On 07/20/2017 03:20 AM, Miklos Szeredi wrote:
> On Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <longman@xxxxxxxxxx> wrote:
>>
>>>> @@ -603,7 +698,13 @@ static struct dentry *dentry_kill(struct dentry *dentry)
>>>>
>>>> if (!IS_ROOT(dentry)) {
>>>> parent = dentry->d_parent;
>>>> - if (unlikely(!spin_trylock(&parent->d_lock))) {
>>>> + /*
>>>> + * Force the killing of this negative dentry when
>>>> + * DCACHE_KILL_NEGATIVE flag is set.
>>>> + */
>>>> + if (unlikely(dentry->d_flags & DCACHE_KILL_NEGATIVE)) {
>>>> + spin_lock(&parent->d_lock);
>>> This looks like d_lock ordering problem (should be parent first, child
>>> second). Why is this needed, anyway?
>>>
>> Yes, that is a bug. I should have used lock_parent() instead.
> lock_parent() can release dentry->d_lock, which means it's perfectly
> useless for this.
As the reference count is kept at 1 in dentry_kill(), the dentry won't
go away even if the dentry lock is temporarily released.
> I still feel forcing free is wrong here. Why not just block until
> the number of negatives goes below the limit (start reclaim if not
> already doing so, etc...)?
Force freeing is the simplest. Any other ways will require adding more
code and increasing code complexity.
One reason why I prefer this is to avoid adding unpredictable latency to
the regular directory lookup and other dentry related operations. We can
always change the code later on if there is a better way of doing it.
Cheers,
Longman
