Re: [PATCH v1 04/14] xprtrdma: Use ib_device pointer safely

Chuck Lever
2015-05-07T14:12:39Z

On May 7, 2015, at 9:56 AM, Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:

> On 5/7/2015 4:39 PM, Chuck Lever wrote:
>>
>> On May 7, 2015, at 6:00 AM, Sagi Grimberg <sagig-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org> wrote:
>>
>>> On 5/4/2015 8:57 PM, Chuck Lever wrote:
>>>> The connect worker can replace ri_id, but prevents ri_id->device
>>>> from changing during the lifetime of a transport instance.
>>>>
>>>> Cache a copy of ri_id->device in rpcrdma_ia and in rpcrdma_rep.
>>>> The cached copy can be used safely in code that does not serialize
>>>> with the connect worker.
>>>>
>>>> Other code can use it to save an extra address generation (one
>>>> pointer dereference instead of two).
>>>>
>>>> Signed-off-by: Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
>>>> ---
>>>> net/sunrpc/xprtrdma/fmr_ops.c | 8 +----
>>>> net/sunrpc/xprtrdma/frwr_ops.c | 12 +++----
>>>> net/sunrpc/xprtrdma/physical_ops.c | 8 +----
>>>> net/sunrpc/xprtrdma/verbs.c | 61 +++++++++++++++++++-----------------
>>>> net/sunrpc/xprtrdma/xprt_rdma.h | 2 +
>>>> 5 files changed, 43 insertions(+), 48 deletions(-)
>>>>
>>>> diff --git a/net/sunrpc/xprtrdma/fmr_ops.c b/net/sunrpc/xprtrdma/fmr_ops.c
>>>> index 302d4eb..0a96155 100644
>>>> --- a/net/sunrpc/xprtrdma/fmr_ops.c
>>>> +++ b/net/sunrpc/xprtrdma/fmr_ops.c
>>>> @@ -85,7 +85,7 @@ fmr_op_map(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr_seg *seg,
>>>> int nsegs, bool writing)
>>>> {
>>>> struct rpcrdma_ia *ia = &r_xprt->rx_ia;
>>>> - struct ib_device *device = ia->ri_id->device;
>>>> + struct ib_device *device = ia->ri_device;
>>>> enum dma_data_direction direction = rpcrdma_data_dir(writing);
>>>> struct rpcrdma_mr_seg *seg1 = seg;
>>>> struct rpcrdma_mw *mw = seg1->rl_mw;
>>>> @@ -137,17 +137,13 @@ fmr_op_unmap(struct rpcrdma_xprt *r_xprt, struct rpcrdma_mr_seg *seg)
>>>> {
>>>> struct rpcrdma_ia *ia = &r_xprt->rx_ia;
>>>> struct rpcrdma_mr_seg *seg1 = seg;
>>>> - struct ib_device *device;
>>>> int rc, nsegs = seg->mr_nsegs;
>>>> LIST_HEAD(l);
>>>>
>>>> list_add(&seg1->rl_mw->r.fmr->list, &l);
>>>> rc = ib_unmap_fmr(&l);
>>>> - read_lock(&ia->ri_qplock);
>>>> - device = ia->ri_id->device;
>>>> while (seg1->mr_nsegs--)
>>>> - rpcrdma_unmap_one(device, seg++);
>>>> - read_unlock(&ia->ri_qplock);
>>>> + rpcrdma_unmap_one(ia->ri_device, seg++);
>>>
>>> Umm, I'm wandering if this is guaranteed to be the same device as
>>> ri_id->device?
>>>
>>> Imagine you are working on a bond device where each slave belongs to
>>> a different adapter. When the active port toggles, you will see a
>>> ADDR_CHANGED event (that the current code does not handle...), what
>>> you'd want to do is just reconnect and rdma_cm will resolve the new
>>> address for you (via the backup slave). I suspect that in case this
>>> flow is concurrent with the reconnects you may end up with a stale
>>> device handle.
>>
>> I’m not sure what you mean by “stale” : freed memory?
>>
>> I’m looking at this code in rpcrdma_ep_connect() :
>>
>> 916 if (ia->ri_id->device != id->device) {
>> 917 printk("RPC: %s: can't reconnect on "
>> 918 "different device!\n", __func__);
>> 919 rdma_destroy_id(id);
>> 920 rc = -ENETUNREACH;
>> 921 goto out;
>> 922 }
>>
>> After reconnecting, if the ri_id has changed, the connect fails. Today,
>> xprtrdma does not support the device changing out from under it.
>>
>> Note also that our receive completion upcall uses ri_id->device for
>> DMA map syncing. Would that also be a problem during a bond failover?
>>
>
> I'm not talking about ri_id->device, this will be consistent. I'm
> wandering about ia->ri_device, which might not have been updated yet.

ia->ri_device is never updated. The only place it is set is in
rpcrdma_ia_open().

> Just asking, assuming your transport device can change between consecutive reconnects (the new cm_id will contain another device), is
> it safe to rely on ri_device being updated?

My reading of the above logic is that ia->ri_id->device is guaranteed to
be the same address during the lifetime of the transport instance. If it
changes during a reconnect, rpcrdma_ep_connect() will fail the connect.

In the case of a bonded device, why are the physical slave devices exposed
to consumers? It might be saner to construct a virtual ib_device in this
case that consumers can depend on.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com

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