linux-kernel - Re: [PATCH 28/30] r/o bind mounts: track numbers of writers to mounts

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Date:	Wed, 20 Feb 2008 13:12:20 -0800
From:	Dave Hansen <haveblue@...ibm.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-kernel@...r.kernel.org, hch@....de, akpm@...l.org
Subject: Re: [PATCH 28/30] r/o bind mounts: track numbers of writers to
	mounts

On Mon, 2008-02-18 at 17:10 +0100, Miklos Szeredi wrote:
> 
> > +     /*
> > +      * We don't have to hold all of the locks at the
> > +      * same time here because we know that we're the
> > +      * last reference to mnt and that no new writers
> > +      * can come in.
> > +      */
> > +     for_each_possible_cpu(cpu) {
> > +             struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
> > +             if (cpu_writer->mnt != mnt)
> > +                     continue;
> > +             spin_lock(&cpu_writer->lock);
> > +             atomic_add(cpu_writer->count, &mnt->__mnt_writers);
> > +             cpu_writer->count = 0;
> 
> I think you should also add a
> 
>                 cpu_writer->mnt = NULL;
> 
> here.  It's not a bug, but I had to think a bit about why it's not a
> bug.

Yeah, I kinda copied the code from __clear_mnt_count() where keeping
->mnt is actually a mini optimization.  But, there is certainly no
chance of that mnt popping up again after a __mntput(), so I clear it in
there now.  I also added a comment explaining.

-- Dave

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