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

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Date:	Tue, 07 Dec 2010 16:49:34 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Nick Piggin <npiggin@...il.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Nick Piggin <npiggin@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/46] rcu-walk and dcache scaling

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/ ?

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.

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.

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