[ewg] [PATCH] IPoIB: synchronize timer deletion with completion handler

[Eli Cohen]

On Wed, Dec 09, 2009 at 02:39:49PM -0800, Roland Dreier wrote:
> 
>  > When calling del_timer_sync on priv->poll_timer, it is necessary to prevent
>  > farther arming of the timer which can be done by a completion handler. Though
>  > it is harmless since the timer will eventually stop being rearmed, it is better
>  > practice to avoid calling the timer function after it is deleted. This patch
>  > handles this by using a new flag that is checked before arming the timer.
> 
> have you seen this in practice?  If it can happen then it's not
> harmless, since the module could be unloaded with the timer pending.
> However I don't see how it could happen, since we only seem to delete
> the timer after we know that no more completions are coming (except for
> the case where we decide that the hardware is wedged but it really only
> takes a *long* time to respond at exactly the wrong time, and we somehow
> get a completion between the del_timer_sync and the modify QP to reset
> state -- which is so unlikely it seems not worth adding this extra
> complexity for -- maybe we could add the del_timer_sync to after we
> delete the CQ or something if you're really worried)

No, I haven't actually seen this happening but I think it could.
Consider this scenario:

1. ipoib_send() calls ib_req_notify_cq()
2. ipoib_ib_dev_stop() gets called
3. All pending WR complete; interrupt is generated but not yet
serviced.
4. ipoib_ib_dev_stop() calls del_timer_sync() and passes it.
5. Completion handler is finally serviced - it schedules
ipoib_ib_tx_timer_func()
6. IPoIB unloads and we're in trouble.

Not so likely to happen indeed. Your suggestion to defer deleting the
timer until after the CQ is destroyed will also solve this.