linux-kernel - Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write acquires

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Date:	Mon, 18 Apr 2011 18:49:38 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 4/7] lockdep: Seperate lock ids for read/write
 acquires

On Mon, 2011-04-18 at 12:46 -0400, Steven Rostedt wrote:
> > +/*
> > + * A lock's class id is used to calculate the chain-key. Since we need to
> > + * differentiate between the chains which contain the read acquire of
> > + * a lock from the chains having write acquire of the same lock,
> > + * we offset the class_idx by MAX_LOCKDEP_KEYS if it is a read acquire.
> 
> Don't we only care to do this if we have a recursive read? I thought
> simple reads still work fine with the current algorithm?
> 
> > + *
> > + * Thus the the lock's key during a chain-key calculation can be in the range
> > + * 1 to 2 * MAX_LOCKDEP_KEYS - 1.
> > + *
> > + * LOCKDEP_CHAIN_KEY_BITS holds the number of bits required to
> > + * represent this range.
> > + */
> > +#define LOCKDEP_CHAIN_KEY_BITS       (MAX_LOCKDEP_KEYS_BITS + 1)
> >  struct held_lock {
> >       /*
> >        * One-way hash of the dependency chain up to this point. We
> > Index: linux-2.6/kernel/lockdep.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/lockdep.c
> > +++ linux-2.6/kernel/lockdep.c
> > @@ -303,8 +303,8 @@ static struct list_head chainhash_table[
> >   * unique.
> >   */
> >  #define iterate_chain_key(key1, key2) \
> > -     (((key1) << MAX_LOCKDEP_KEYS_BITS) ^ \
> > -     ((key1) >> (64-MAX_LOCKDEP_KEYS_BITS)) ^ \
> > +     (((key1) << LOCKDEP_CHAIN_KEY_BITS) ^ \
> > +     ((key1) >> (64 - LOCKDEP_CHAIN_KEY_BITS)) ^ \
> >       (key2))
> >  
> >  void lockdep_off(void)
> > @@ -1988,6 +1988,9 @@ static void check_chain_key(struct task_
> >               if (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
> >                       return;
> >  
> > +             if (is_read(hlock->rw_state))
> > +                     id += MAX_LOCKDEP_KEYS;
> 
> Again, isn't this about recursive reads? Or am I just confused ;) 

So what we do here is split off the write chain, the above could have
been writeen if (!is_write()) to clarify that.

Everything except recursive read validation will traverse both chains,
the recursive read validation will only traverse the write chains and
ignore the combined read/recursive-read chain.
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