Re: [PATCH 00/46] rcu-walk and dcache scaling

[Nick Piggin]

On Wed, Dec 8, 2010 at 2:49 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Wed, 2010-12-08 at 02:24 +1100, Nick Piggin wrote:
>>
>>  repeat:
>>     spin_lock(&parent->d_lock);
>>     spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
>>     /* do stuff */
>>     spin_unlock(&parent->d_lock);
>>     spin_release(&dentry->d_lock.dep_map, 1, _RET_IP_);
>>     parent = dentry;
>>     spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
>>     goto repeat;
>
> shouldn't that be s/this_parent/parent/ ?

Yes, typo in my pseudo code.


> So what you're trying to do is:
>
>  A -> B -> C -> ...
>
> lock A
> lock B, nested
> unlock A
> flip B from nested to top
> lock C, nested
> unlock B
> flip C from nested to top
> lock ...
>
> Anyway, the way to write that is something like:
>
>  lock_set_subclass(&detry->d_lock.dep_map, 0, _RET_IP_);
>
> Which will reset the subclass of the held lock from DENTRY_D_LOCK_NESTED
> to 0.

OK, thanks. My version should not have caused any problems though,
right? Any idea what might have caused Dave's crash?


> This is also used in double_unlock_balance(), we go into
> double_lock_balance() with this_rq locked and want to lock busiest,
> because of the lock ordering we might need to unlock this_rq and lock
> busiest first, at which point this_rq is nested.
>
> On unlock we thus need to map this_rq back to subclass 0 (which it had
> before double_lock_balance(), because otherwise subsequent lock
> operations will be done against the subclass and confuse things.

Thanks,
Nick
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