linux-kernel - Re: [PATCH 12/25] aio: Refcounting cleanup

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Message-ID: <20121129011203.GC15094@google.com>
Date:	Wed, 28 Nov 2012 17:12:03 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Zach Brown <zab@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, bcrl@...ck.org, jmoyer@...hat.com,
	axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 12/25] aio: Refcounting cleanup

On Wed, Nov 28, 2012 at 04:17:59PM -0800, Zach Brown wrote:
> 
> >  struct kioctx {
> >  	atomic_t		users;
> > -	int			dead;
> > +	atomic_t		dead;
> 
> Do we want to be paranoid and atomic_set() that to 0 when the ioctx is
> allocated?

I suppose we should, yeah.

> > +	while (!list_empty(&ctx->active_reqs)) {
> > +		struct list_head *pos = ctx->active_reqs.next;
> > +		struct kiocb *iocb = list_kiocb(pos);
> 
> I'd use list_first_entry() and ignore the list_kiocb() wrapper, I think.

Fine by me.

> 
> > +	if (!atomic_xchg(&ctx->dead, 1)) {
> > +		hlist_del_rcu(&ctx->list);
> > +		synchronize_rcu();
> 
> > +	hlist_for_each_entry_rcu(ctx, n, &mm->ioctx_list, list)
> > +		if (ctx->user_id == ctx_id){
> > +			BUG_ON(atomic_read(&ctx->dead));
> 
> Hmm.  Won't it be possible to race lookup and io_destroy() to hit this
> BUG?

Ouch, yes. Dunno what I was thinking when I added that.
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