On Tue, 2018-07-10 at 22:44 +0200, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
> On Tue, Jul 10, 2018 at 08:23:01PM +0000, Bart Van Assche wrote:
> > On Tue, 2018-07-10 at 20:51 +0200, Greg KH wrote:
> > > On Tue, Jul 10, 2018 at 10:32:00AM -0700, Bart Van Assche wrote:
> > > > diff --git a/drivers/infiniband/ulp/srpt/ib_srpt.c b/drivers/infiniband/ulp/srpt/ib_srpt.c
> > > > index 325bae29e90d..705f6a992d82 100644
> > > > --- a/drivers/infiniband/ulp/srpt/ib_srpt.c
> > > > +++ b/drivers/infiniband/ulp/srpt/ib_srpt.c
> > > > @@ -2152,6 +2152,7 @@ static int srpt_cm_req_recv(struct srpt_device *const sdev,
> > > > }
> > > >
> > > > kref_init(&ch->kref);
> > > > + kref_get(&ch->kref);
> > >
> > > kref_init starts the reference count at at 1, so why do you need to
> > > increment it right away? That feels like something is "odd" here, why
> > > do you start with 2 references in the same function?
> >
> > An ib_srpt RDMA channel object (ch in the above code) must stay around as long
> > as the associated target core session (se_sess) exists and also as long as the
> > target core has not yet called srpt_close_session(). Hence the initialization of
> > ch->kref to 2 just before an RDMA channel is registered with the target core.
>
> Shouldn't the registration increment the reference? Starting out at "2"
> feels very "odd", don't you agree?
Hello Greg,
The code that registers the session with the target core is in another driver
(SCSI target core) and that driver doesn't know about the abstractions maintained
by the ib_srpt driver. That's why the above kref_get() call is in the ib_srpt
driver and not e.g. in the target_alloc_session() function.
BTW, there is more code in the Linux kernel that follows the above pattern.
target_submit_cmd_map_sgls() initializes the se_cmd reference count to two as
follows:
* transport_init_se_cmd() initializes it to one.
* target_get_sess_cmd() increments it from one to two.
This is because two contexts keep a reference to se_cmd data structures,
namely the target core and the target driver. A SCSI target command data
structure (se_cmd) must only be freed after both contexts have finished their
part of the command processing.
Thanks,
Bart.
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