Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown <neilb@suse.de>]

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On Wed, 20 May 2015 06:28:35 -0700 "Paul E. McKenney"
<paulmck@linux.vnet.ibm.com> wrote:

> On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote:
> > On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
> > <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > 
> > > The code in md probably needs to change in any case, as otherwise we are
> > > invoking rcu_dereference_whatever() on a full struct list_head rather
> > > than on a single pointer.  Or am I missing something here?
> > 
> > I think it would be
> >    rcu_dereference_whatever(&mddev->disks)
> > 
> > I don't know what you mean by "on a full struct list_head", but there is
> > nothing actually being dereferenced here - right?  Just pointer arithmetic on
> > 'mddev'.
> 
> It really does dereference.  Strange but true.

Well... your the expert.  But without an lvalue, I can't see it.


> 
> > I should probably just
> > 
> > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> > index 2bc56e2a3526..b1d237bf8b3b 100644
> > --- a/drivers/md/bitmap.c
> > +++ b/drivers/md/bitmap.c
> > @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
> >  	rcu_read_lock();
> >  	if (rdev == NULL)
> >  		/* start at the beginning */
> > -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> > +		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
> >  	else {
> >  		/* release the previous rdev and start from there. */
> >  		rdev_dec_pending(rdev, mddev);
> > 
> > as there really are no RCU issues with getting that address.  Maybe I should
> > move it outside the rcu_read_lock() just to be blatant.... but that would
> > make the code a lot more clumsy as the rdev_dec_pending must be inside the
> > rcu_read_lock..
> > 
> > So this.
> 
> Fair enough -- if you aren't using RCU, there is really no point in using
> the RCU API.  I will drop this patch from my tree.  You are pushing yours,
> I am guessing?

Excellent guess :-)
Hopefully for the next -rc.

Thanks,
NeilBrown


> 
> 							Thanx, Paul
> 
> > Thanks,
> > NeilBrown
> > 
> > From: NeilBrown <neilb@suse.de>
> > Date: Wed, 20 May 2015 15:05:09 +1000
> > Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic.
> > 
> > Evaluating  "&mddev->disks" is simple pointer arithmetic, so
> > it does not need 'rcu' annotations - no dereferencing is happening.
> > 
> > Also enhance the comment to explain that 'rdev' in that case
> > is not actually a pointer to an rdev.
> > 
> > Reported-by: Patrick Marlier <patrick.marlier@gmail.com>
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > 
> > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> > index 2bc56e2a3526..135a0907e9de 100644
> > --- a/drivers/md/bitmap.c
> > +++ b/drivers/md/bitmap.c
> > @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
> >  	 * nr_pending is 0 and In_sync is clear, the entries we return will
> >  	 * still be in the same position on the list when we re-enter
> >  	 * list_for_each_entry_continue_rcu.
> > +	 *
> > +	 * Note that if entered with 'rdev == NULL' to start at the
> > +	 * beginning, we temporarily assign 'rdev' to an address which
> > +	 * isn't really an rdev, but which can be used by
> > +	 * list_for_each_entry_continue_rcu() to find the first entry.
> >  	 */
> >  	rcu_read_lock();
> >  	if (rdev == NULL)
> >  		/* start at the beginning */
> > -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> > +		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
> >  	else {
> >  		/* release the previous rdev and start from there. */
> >  		rdev_dec_pending(rdev, mddev);
> 


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