Re: [PATCH for-next v4 0/2] RDMA/efa: Add AH cache for AH reuse

[Leon Romanovsky <leon@kernel.org>]

On Sun, Jun 14, 2026 at 07:12:29AM +0000, Yonatan Nachum wrote:
> On Mon, Jun 08, 2026 at 07:16:18AM +0000, Yonatan Nachum wrote:
> > Changelog:
> > v4:
> >  * Use kzalloc_obj for AH cache entry allocation instead of kzalloc
> > v3: https://lore.kernel.org/all/20260607161753.1607559-1-ynachum@amazon.com/
> >  * Address Sashiko comments in:
> >    https://sashiko.dev/#/patchset/20260512061121.2177521-1-ynachum%40amazon.com
> > v2: https://lore.kernel.org/all/20260512061121.2177521-1-ynachum@amazon.com/
> >  * Zero-initialize AH cache key on cache lookup.
> > v1: https://lore.kernel.org/all/20260510083035.458081-1-ynachum@amazon.com/
> > 
> > -------------------------------------------------------------------------
> > New EFA devices don't support the creation of multiple AHs to the same
> > remote on the same PD. To overcome this limitation, introduce an AH
> > cache that manages AH reuse transparently.
> > 
> > The cache uses an rhashtable keyed by (PD, GID) to track active address
> > handles with refcounts. On create AH, the driver returns an existing AH
> > number if one is already cached, or creates a new one and caches it. On
> > destroy AH, the driver only issues the device destroy command when the
> > last reference is dropped.
> > 
> > A per-entry mutex serializes concurrent device commands on the same
> > cache entry, preventing create-before-destroy races on the device.
> > 
> > Yonatan Nachum (2):
> >   RDMA/efa: Add initialization of AH cache rhashtable
> >   RDMA/efa: Add AH cache handling on create and destroy AH
> > 
> >  drivers/infiniband/hw/efa/Makefile       |   4 +-
> >  drivers/infiniband/hw/efa/efa_ah_cache.c | 163 +++++++++++++++++++++++
> >  drivers/infiniband/hw/efa/efa_ah_cache.h |  42 ++++++
> >  drivers/infiniband/hw/efa/efa_com.c      |  12 +-
> >  drivers/infiniband/hw/efa/efa_com.h      |   3 +
> >  drivers/infiniband/hw/efa/efa_com_cmd.c  |  73 +++++++---
> >  drivers/infiniband/hw/efa/efa_com_cmd.h  |   1 +
> >  drivers/infiniband/hw/efa/efa_verbs.c    |   9 +-
> >  8 files changed, 281 insertions(+), 26 deletions(-)
> >  create mode 100644 drivers/infiniband/hw/efa/efa_ah_cache.c
> >  create mode 100644 drivers/infiniband/hw/efa/efa_ah_cache.h
> > 
> > -- 
> > 2.50.1
> >
> 
> Hi, kind reminder.
> This series blocks merging EFAv4 device ID and we would want it to land
> for the next merge window if possible.

It is not possible.

The use of entry->lock together with a refcount and an "initialize"
flag suggests that the refcount is not being used correctly.

I would expect a single ah_cache lock, with the refcount tracking the
number of users of the entry.

Thanks.

> 
> I reviewed the last Sashiko comment and it's not relevant since on
> destroy AH failure, we keep the entry in the hashtable.
> 
> Thanks
>