[openib-general] [PATCH] RDMA/iwcm: Get rid of extra call to list_empty()

[Sean Hefty]

Tom Tucker wrote:
> This patch adds a race that causes an event to be processed twice. 
> 
> Each cm_id has a list of pending work stuck on cm_id_priv->work_list.
> When new work is created, cm_event_handler needs to know whether it
> needs to schedule a call to cm_work_handler by enqueing work on the
> iwcm_wq or whether it can just append it to the list of work already
> scheduled. 
> 
> The race is that the cm_work_handler removes the last element on the
> list (leaving it empty) and then releases the lock. The cm_event_handler
> checks the list, finds it empty and so schedules another call to
> cm_work_handler (which is still running). cm_work_handler processes the
> work (previous end of list) and then takes the lock again, checks the
> list and voila, finds the work just added by cm_event_handler, so he
> processes it. But the work is still queued on the iwcm_wq. So when the
> cm_work_handler runs again, it processes that same work element a second
> time -- causing all kinds of misadventure.

There may be a race here, but...  Why wouldn't the second call into 
cm_work_handler simply find the list empty on entry into the call?  As an 
alternative, could you defer the list_del_init() call to the end of the loop, 
which would avoid scheduling cm_work_handler while it's running?

- Sean