linux-kernel - Re: [patch 44/52] fs: icache per-CPU sb inode lists and locks

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Date:	Thu, 1 Jul 2010 18:00:43 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: [patch 44/52] fs: icache per-CPU sb inode lists and locks

On Thu, Jul 01, 2010 at 01:12:00PM +1000, Dave Chinner wrote:
> On Wed, Jun 30, 2010 at 10:08:50PM +1000, Nick Piggin wrote:
> > > I can't say that I'm a great fan of hiding loop context in defines
> > > like this. It reminds far too much of how parts of Irix slowly
> > > ossified because they ended up mess of complex, fragile macros that
> > > nobody fully understood...
> > 
> > It's not perfect. I think it is a lot better than open coding
> > (which I tried before) because that really muddies up the intention
> > of the loop body.
> 
> Something like this doesn't seem particularly bad:
> 
> static inline struct list_head *
> inode_get_sb_list(struct super_block *sb, int *i)
> {
> 	int cpu;
> 
> 	cpu = cpumask_next(i, cpu_possible_mask);
> 	if (cpu >= nr_cpu_ids)
> 		return NULL;
> 	*i = cpu;
> #ifdef CONFIG_SMP
> 	return per_cpu_ptr(sb->s_inodes, cpu);
> #else
> 	return &sb->s_inodes;
> #endif
> }
> 
> and:
> 
> 	struct list_head *list;
> 	int i;
> ....
> 	i = -1;
> 	while ((list = inode_get_sb_list(sb, &i))) {
> 		list_for_each_entry_rcu(inode, tmp, list, i_sb_list) {
> 			.....
> 		}
> 	}
> 
> I'd much prefer this to hiding the outer loop in macros...

I don't see it as an improvement. A macro called
inode_sb_list_for_each_entry is rather obvious and idiomatic as to what
it does. You don't need to look at the implementation if you just need
to walk the inode sb list.

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