rds cq event handler issue

[Steve Wise <swise-7bPotxP6k4+P2YhJcF5u+vpXobYPEAuW@public.gmane.org>]

Hey Venkat,

I think I see a bug in the RDS RDMA module where RDS is not adhering to the RDMA locking context.  From the kernel tree 
Documentation/infiniband/core_locking.txt:

---
   The context in which completion event and asynchronous event
   callbacks run is not defined.  Depending on the low-level driver, it
   may be process context, softirq context, or interrupt context.
   Upper level protocol consumers may not sleep in a callback.
---

So RDMA ULPs cannot assume any certain context for their callback functions.  Yet I get a BUG_ON() when running RDS with 
iw_cxgb3 where RDS is bugging in rds_rdma_free_op():

---
                 /* Mark page dirty if it was possibly modified, which
                  * is the case for a RDMA_READ which copies from remote
                  * to local memory */
                 if (!ro->op_write) {
                         BUG_ON(irqs_disabled());
                         set_page_dirty(page);
                 }
---

And rds_rdma_free_op() can be called in the cq callback path.  Here's a stack trace when it bugged:

---
Call Trace:
<IRQ> [<ffffffff886ca0fc>] :rds:rds_message_purge+0x54/0x79
[<ffffffff886ca162>] :rds:rds_message_put+0x41/0x4c
[<ffffffff886f616b>] :rds_rdma:rds_iw_send_unmap_rm+0xe2/0xf2
[<ffffffff886f63c4>] :rds_rdma:rds_iw_send_cq_comp_handler+0x193/0x2e5
[<ffffffff88698a56>] :iw_cxgb3:iwch_ev_dispatch+0x1df/0x2b1
[<ffffffff8869f0b2>] :iw_cxgb3:cxio_hal_ev_handler+0x6b/0xb4
[<ffffffff882746cd>] :cxgb3:process_rx+0x3d/0xa0
[<ffffffff8827b28c>] :cxgb3:process_responses+0x120c/0x1350
---

iwch_ev_dispatch() explicitly disables irqs to ensure proper serialization:

---
         spin_lock_irqsave(&chp->comp_handler_lock, flag);
         (*chp->ibcq.comp_handler)(&chp->ibcq, chp->ibcq.cq_context);
         spin_unlock_irqrestore(&chp->comp_handler_lock, flag);
---

I'm not sure if that BUG_ON() in rds_rdma_free_op() is valid or not.  If it is valid, then RDS needs to run this logic 
in a safe context, not in the context of the CQ callback.  It BUG_ON() is not valid, we can remove it :).

Can you comment?

Thanks,

Steve.

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