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
