Date: Thu, 6 Sep 2012 07:55:46 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Pedro Alves <palves@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        Tejun Heo <tj@kernel.org>, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, paul.gortmaker@windriver.com, davem@davemloft.net,
        mingo@elte.hu, ebiederm@xmission.com, aarcange@redhat.com,
        ericvh@gmail.com, netdev@vger.kernel.org, eric.dumazet@gmail.com,
        axboe@kernel.dk, agk@redhat.com, dm-devel@redhat.com, neilb@suse.de,
        ccaulfie@redhat.com, teigland@redhat.com, Trond.Myklebust@netapp.com,
        bfields@fieldses.org, fweisbec@gmail.com, jesse@nicira.com,
        venkat.x.venkatsubra@oracle.com, ejt@redhat.com, snitzer@redhat.com,
        edumazet@google.com, linux-nfs@vger.kernel.org, dev@openvswitch.org,
        rds-devel@oss.oracle.com, lw@cn.fujitsu.com
Subject: Re: [PATCH v3 01/17] hashtable: introduce a small and naive hashtable
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On Thu, Sep 06, 2012 at 03:53:58PM +0200, Sasha Levin wrote:
> On 09/04/2012 07:01 PM, Mathieu Desnoyers wrote:
> >> #define do_for_each_ftrace_rec(pg, rec)                                          \
> >> >         for (pg = ftrace_pages_start, rec = &pg->records[pg->index];             \
> >> >              pg && rec == &pg->records[pg->index];                               \
> >> >              pg = pg->next)                                                      \
> >> >           for (rec = pg->records; rec < &pg->records[pg->index]; rec++)
> > Maybe in some cases there might be ways to combine the two loops into
> > one ? I'm not seeing exactly how to do it for this one, but it should
> > not be impossible. If the inner loop condition can be moved to the outer
> > loop, and if we use (blah ? loop1_conf : loop2_cond) to test for
> > different conditions depending on the context, and do the same for the
> > 3rd argument of the for() loop. The details elude me for now though, so
> > maybe it's complete non-sense ;)
> > 
> > It might not be that useful for do_for_each_ftrace_rec, but if we can do
> > it for the hash table iterator, it might be worth it.
> 
> So I think that for the hash iterator it might actually be simpler.
> 
> My solution to making 'break' work in the iterator is:
> 
> 	for (bkt = 0, node = NULL; bkt < HASH_SIZE(name) && node == NULL; bkt++)
> 		hlist_for_each_entry(obj, node, &name[bkt], member)
> 
> We initialize our node loop cursor with NULL in the external loop, and the
> external loop will have a new condition to loop while that cursor is NULL.
> 
> My logic is that we can only 'break' when we are iterating over an object in the
> internal loop. If we're iterating over an object in that loop then 'node != NULL'.
> 
> This way, if we broke from within the internal loop, the external loop will see
> node as not NULL, and so it will stop looping itself. On the other hand, if the
> internal loop has actually ended, then node will be NULL, and the outer loop
> will keep running.
> 
> Is there anything I've missed?

Looks reasonable.  However, it would break (or rather, not break) on
code like this:

	hash_for_each_entry(...) {
		if (...) {
			foo(node);
			node = NULL;
			break;
		}
	}

Hiding the double loop still seems error-prone.

- Josh Triplett