Re: [PATCH][RFC]Ext4: Use get_cpu()/put_cpu() in preemptible context

[Mingming Cao <cmm@us.ibm.com>]

On Sat, 2007-11-03 at 13:01 +0800, Andreas Dilger wrote:
> On Nov 02, 2007  17:35 -0700, Mingming Cao wrote:
> > Index: linux-2.6.24-rc1/fs/ext4/mballoc.c
> > ===================================================================
> > --- linux-2.6.24-rc1.orig/fs/ext4/mballoc.c	2007-11-02 17:22:18.000000000 -0700
> > +++ linux-2.6.24-rc1/fs/ext4/mballoc.c	2007-11-02 17:23:02.000000000 -0700
> > @@ -4006,7 +4006,8 @@ static void ext4_mb_group_or_file(struct
> >  		return;
> >  
> >  	BUG_ON(ac->ac_lg != NULL);
> > -	ac->ac_lg = &sbi->s_locality_groups[smp_processor_id()];
> > +	ac->ac_lg = &sbi->s_locality_groups[get_cpu()];
> > +	put_cpu();
> >  
> >  	/* we're going to use group allocation */
> >  	ac->ac_flags |= EXT4_MB_HINT_GROUP_ALLOC;
> 
> Shouldn't the put_cpu() be after ac->ac_lg is no longer being used?
> I guess there would otherwise be a danger of other processes using
> the same s_locality_groups[] struct?

>From the code, the concurrent use of the same s_locality_groups is being
protected by the ac_lg->lg_sem. The put_cpu() instruction is before the
lock is taken.


Mingming