linux-kernel - Re: [PATCH 1/2] fs/ocfs2/dlm: Eliminate update of list_for_each

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Message-ID: <20111102073928.GA23750@mwanda>
Date:	Wed, 2 Nov 2011 10:39:28 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Julia Lawall <julia@...u.dk>, Mark Fasheh <mfasheh@...e.com>,
	ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Sunil Mushran <sunil.mushran@...cle.com>
Subject: Re: [PATCH 1/2] fs/ocfs2/dlm: Eliminate update of
 list_for_each_entry loop cursor

What ever happened with this?  The bug is still there in the latest
kernel.

I think from previous discussion about this that we only ever have
one lock so lock->ml.cookie is always equal to ml->cookie and we
never set lock to NULL.  So we never actually hit the NULL deref.
But it should probably still be cleaned up.

regards,
dan carpenter

On Wed, Aug 11, 2010 at 05:03:56PM -0700, Joel Becker wrote:
> On Sat, Aug 07, 2010 at 11:09:13AM +0200, Julia Lawall wrote:
> > From: Julia Lawall <julia@...u.dk>
> > 
> > list_for_each_entry uses its first argument to move from one element to the
> > next, so modifying it can break the iteration.
> 
> 	Thanks for catching the bug.  It was introduced by 800deef3
> [ocfs2: use list_for_each_entry where benefical].  I blame Christoph.
> 
> > diff --git a/fs/ocfs2/dlm/dlmrecovery.c b/fs/ocfs2/dlm/dlmrecovery.c
> > index 9dfaac7..7084a11 100644
> > --- a/fs/ocfs2/dlm/dlmrecovery.c
> > +++ b/fs/ocfs2/dlm/dlmrecovery.c
> > @@ -1792,10 +1792,10 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
> >  			for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
> >  				tmpq = dlm_list_idx_to_ptr(res, j);
> >  				list_for_each_entry(lock, tmpq, list) {
> > -					if (lock->ml.cookie != ml->cookie)
> > +					if (lock->ml.cookie != ml->cookie) {
> >  						lock = NULL;
> > -					else
> >  						break;
> > +					}
> >  				}
> >  				if (lock)
> >  					break;
> 
> 	However, this is not the correct solution.  The goal of the
> original code, which used to use list_for_each(), was to leave lock
> non-NULL if the cookie was found.  Your version merely exits the loop on
> the first non-matching entry, always leaving lock==NULL if there is a
> non-matching entry.
> 	One possible solution is to return the original code:
> 
> --8<-----------------------------------------------------------------
> @@ -1747,7 +1747,7 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
>  				     struct dlm_migratable_lockres *mres)
>  {
>  	struct dlm_migratable_lock *ml;
> -	struct list_head *queue;
> +	struct list_head *queue, *iter;
>  	struct list_head *tmpq = NULL;
>  	struct dlm_lock *newlock = NULL;
>  	struct dlm_lockstatus *lksb = NULL;
> @@ -1791,11 +1791,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
>  			spin_lock(&res->spinlock);
>  			for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
>  				tmpq = dlm_list_idx_to_ptr(res, j);
> -				list_for_each_entry(lock, tmpq, list) {
> -					if (lock->ml.cookie != ml->cookie)
> -						lock = NULL;
> -					else
> +				list_for_each(iter, tmpq) {
> +					lock = list_entry(iter, struct dlm_lock, list);
> +
> +					if (lock->ml.cookie == ml->cookie)
>  						break;
> +					lock = NULL;
>  				}
>  				if (lock)
>  					break;
> -->8-----------------------------------------------------------------
> 
> 	Another approach would be to keep list_for_each_entry() around,
> but use a better check for entry existence:
> 
> --8<-----------------------------------------------------------------
> @@ -1792,13 +1792,12 @@ static int dlm_process_recovery_data(struct dlm_ctxt *dlm,
>  			for (j = DLM_GRANTED_LIST; j <= DLM_BLOCKED_LIST; j++) {
>  				tmpq = dlm_list_idx_to_ptr(res, j);
>  				list_for_each_entry(lock, tmpq, list) {
> -					if (lock->ml.cookie != ml->cookie)
> -						lock = NULL;
> -					else
> +					if (lock->ml.cookie == ml->cookie)
>  						break;
>  				}
> -				if (lock)
> +				if (&lock->list != tmpq)
>  					break;
> +				lock = NULL;
>  			}
>  
>  			/* lock is always created locally first, and
> -->8-----------------------------------------------------------------
> 
> 	I think I like the second one better.  Sunil, what do you think?
> 
> Joel
> 
> -- 
> 
> Life's Little Instruction Book #335
> 
> 	"Every so often, push your luck."
> 
> Joel Becker
> Consulting Software Developer
> Oracle
> E-mail: joel.becker@...cle.com
> Phone: (650) 506-8127
> --
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