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drain CQ after moving QP into RESET state - #3261
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Otherwise there might segfault due to the race below:
```txt
Socket::OnInputEvent() |
`-- ProcessEvent (bthread) |
|
[ bthread queueed ] | QP error -> SetFailed -> HC -> WaitAndReset()
| Reset() -> _sbuf.clear()
| CheckHealth() -> Revive()
|
| Socket is now Addressable!
RdmaEndpoint:PollCq() |
Socket::Address() OK! |
RdmaEndpoint:HandleCompletion()
_sbuf[_sq_sent++].clear() <= BOOM! CQ is not drained but _sbuf is cleared.
```
Another possible fix is to add a _generation_ field in RdmaEndpoint, such that:
- each RdmaEndpoint::Reset() will advance the _generation_ by 1;
- the RdmaEndpoint::PollCq(m, orig_gen) will need to compare the _generation_.
But it will contaminate existing interface, and we need to drain CQ anyway.
Signed-off-by: David Lee <live4thee@gmail.com>
Contributor
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Pull request overview
This PR addresses an RDMA reset/revive race where stale CQEs can be consumed after a QP is moved to RESET and internal send buffers are cleared, leading to a crash (issue #3252). It ensures completion queues are drained before reusing a prepared QP, and falls back to reclaiming resources if draining fails.
Changes:
- Add a
DrainCq()helper to poll and clear CQEs from a CQ. - Drain polling/send/recv CQs after moving the QP to
RESETand before returning the resource to the prepared-QP list. - If draining fails, do not reuse the QP; instead reclaim/destroy resources via the existing cleanup path.
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| ret = ibv_poll_cq(cq, 1, &wc); | ||
| } while (ret > 0); | ||
|
|
||
| LOG_IF(ERROR, ret < 0) << "drain CQ failed: " << ret; |
Comment on lines
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to
+1419
| int ret = DrainCq(_resource->polling_cq); | ||
| ret += DrainCq(_resource->send_cq); | ||
| ret += DrainCq(_resource->recv_cq); | ||
| if (ret < 0) { | ||
| move_to_rdma_resource_list = false; | ||
| goto _reclaim; | ||
| } |
Comment on lines
+1403
to
+1418
| // When a QP is moved to the RESET state, all associated send and | ||
| // receive queues are flushed, meaning any outstanding WRs are effectively | ||
| // abandoned by the hardware. | ||
| // | ||
| // However, the CQ associated with that QP is *not* cleared automatically, | ||
| // meaning that it will still contain entries for WRs that completed before | ||
| // the reset. | ||
| // | ||
| // The application should finish polling the CQ to remove these obsolete | ||
| // entries before reusing the QP. | ||
| int ret = DrainCq(_resource->polling_cq); | ||
| ret += DrainCq(_resource->send_cq); | ||
| ret += DrainCq(_resource->recv_cq); | ||
| if (ret < 0) { | ||
| move_to_rdma_resource_list = false; | ||
| goto _reclaim; |
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Otherwise there might segfault due to the race below:
Another possible fix is to add a generation field in RdmaEndpoint, such that:
But it will contaminate existing interface, and we need to drain CQ anyway.
What problem does this PR solve?
Issue Number: 3252
Problem Summary: see above.
What is changed and the side effects?
Changed:
Side effects:
Performance effects: n/a
Breaking backward compatibility: none
Check List: