Re: [PATCH for-next v13 03/10] RDMA/rxe: Check rxe_get() return value

[Bob Pearson <rpearsonhpe@gmail.com>]

On 4/8/22 12:52, Jason Gunthorpe wrote:
> On Mon, Apr 04, 2022 at 04:50:53PM -0500, Bob Pearson wrote:
>> In the tasklets (completer, responder, and requester) check the
>> return value from rxe_get() to detect failures to get a reference.
>> This only occurs if the qp has had its reference count drop to
>> zero which indicates that it no longer should be used. This is
>> in preparation to an upcoming change that will move the qp cleanup
>> code to rxe_qp_cleanup().
> 
> These need some comments explaining how this is safe..
> 
> It looks to me like it works because the 0 ref keeps the memory alive
> while a work queue triggers rxe_cleanup_task() (though who fences the
> responder task?)
> 
> At least after the next patch, I'm a little unclear how this works
> at this moment..
> 
> Jason

I started writing the comment (here)

    If rxe_get() fails qp is not going to be around for long

    because its ref count has gone to zero and rxe_complete()

    is cleaning up and returning to rdma-core which will

    free the qp. However rxe_do_qp_cleanup() has to finish first

    and it will wait for the tasklets to finish running.

    This fixes a hard bug to solve since the code calling

    rxe_run_task() will hold a valid reference on qp but the

    tasklet can be deferred until later and that reference may

    be gone when the tasklet starts.
 


but I realized that at the end of the day there isn't a problem because
complete/wait_for_completion together with qp cleanup code shutting down
the tasklets means that the race won't happen once the series is all in
place. So I will just drop that patch.

Bob