Re: [PATCH 04/10] Protect kref_put with the lock

[Maksym Planeta]

On 22/07/2019 17:25, Jason Gunthorpe wrote:
> On Mon, Jul 22, 2019 at 05:14:20PM +0200, Maksym Planeta wrote:
> > Need to ensure that kref_put does not run concurrently with the loop
> > inside rxe_pool_get_key.
> >
> > Signed-off-by: Maksym Planeta <mplaneta@xxxxxxxxxxxxxxxxxxxx>
> >   drivers/infiniband/sw/rxe/rxe_pool.c | 18 ++++++++++++++++++
> >   drivers/infiniband/sw/rxe/rxe_pool.h |  4 +---
> >   2 files changed, 19 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/infiniband/sw/rxe/rxe_pool.c b/drivers/infiniband/sw/rxe/rxe_pool.c
> > index efa9bab01e02..30a887cf9200 100644
> > +++ b/drivers/infiniband/sw/rxe/rxe_pool.c
> > @@ -536,3 +536,21 @@ void *rxe_pool_get_key(struct rxe_pool *pool, void *key)
> >   	read_unlock_irqrestore(&pool->pool_lock, flags);
> >   	return node ? elem : NULL;
> >   }
> > +
> > +static void rxe_dummy_release(struct kref *kref)
> > +{
> > +}
> > +
> > +void rxe_drop_ref(struct rxe_pool_entry *pelem)
> > +{
> > +	int res;
> > +	struct rxe_pool *pool = pelem->pool;
> > +	unsigned long flags;
> > +
> > +	write_lock_irqsave(&pool->pool_lock, flags);
> > +	res = kref_put(&pelem->ref_cnt, rxe_dummy_release);
> > +	write_unlock_irqrestore(&pool->pool_lock, flags);
>
> This doesn't make sense..
>
> If something is making the kref go to 0 while the node is still in the
> RB tree then that is a bug.
>
> You should never need to add locking around a kref_put.

From https://www.kernel.org/doc/Documentation/kref.txt

| The last rule (rule 3) is the nastiest one to handle.  Say, for
| instance, you have a list of items that are each kref-ed, and you wish
| to get the first one.  You can't just pull the first item off the list
| and kref_get() it.  That violates rule 3 because you are not already
| holding a valid pointer.  You must add a mutex (or some other lock).


> Jason

--
Regards,
Maksym Planeta