BLueStore Deadlock

Commit Message

Somnath Roy July 28, 2016, 10:21 p.m. UTC

Jianpeng,
I thought through this and it seems there could be one possible deadlock scenario.

tp_osd_tp --> waiting on onode->flush() for previous txc to finish. Holding Wlock(coll)
aio_complete_thread --> waiting for RLock(coll)
No other thread will be blocked here.

We do add previous txc in the flush_txns list during _txc_write_nodes() and before aio_complete_thread calling _txc_state_proc(). So, if within this time frame if we have IO on the same collection , it will be waiting on unfinished txcs.
The solution to this could be the following..

root@emsnode5:~/ceph-master/src# git diff
I am not able to reproduce this in my setup , so, if you can do the above changes in your env and see if you are still hitting the issue, would be helpful.

Thanks & Regards
Somnath


-----Original Message-----
From: Somnath Roy
Sent: Thursday, July 28, 2016 8:45 AM
To: 'Ma, Jianpeng'
Cc: ceph-devel
Subject: RE: BLueStore Deadlock

Hi Jianpeng,
Are you trying with latest master and still hitting the issue (seems so but confirming) ?
The following scenario should not be creating deadlock because of the following reason.

Onode->flush() is waiting on flush_lock() and from _txc_finish() it is releasing that before taking osr->qlock(). Am I missing anything ?

I got a deadlock in this path in one of my earlier changes in the following pull request (described in detail there) and it is fixed and merged.

https://github.com/ceph/ceph/pull/10220

If my theory is right , we are hitting deadlock because of some other reason may be. It seems you are doing WAL write , could you please describe the steps to reproduce ?

Thanks & Regards
Somnath

From: Ma, Jianpeng [mailto:[e-mail]]
Sent: Thursday, July 28, 2016 1:46 AM
To: Somnath Roy
Cc: ceph-devel; Ma, Jianpeng
Subject: BLueStore Deadlock

Hi Roy:
     When do seqwrite w/ rbd+librbd, I met deadlock for bluestore. It can reproduce 100%.(based on 98602ae6c67637dbadddd549bd9a0035e5a2717)
By add message and this found this bug caused by bf70bcb6c54e4d6404533bc91781a5ef77d62033.
Consider this case:

tp_osd_tp                                              aio_complete_thread                                 kv_sync_thread
Rwlock(coll)                                            txc_finish_io                                                     _txc_finish do_write                                                 lock(osr->qlock)                                               lock(osr->qlock) do_read                                                  RLock(coll)                                                                     need osr->qlock to continue
onode->flush()                                            need coll
onode->readlock to continue
   need previous txc complete

But current I don't how to fix this.

Thanks!
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Patch

diff --git a/src/os/bluestore/BlueStore.cc b/src/os/bluestore/BlueStore.cc
index e8548b1..575a234 100644
--- a/src/os/bluestore/BlueStore.cc
+++ b/src/os/bluestore/BlueStore.cc
@@ -4606,6 +4606,11 @@  void BlueStore::_txc_state_proc(TransContext *txc)
         (txc->first_collection)->lock.get_read();
       }
       for (auto& o : txc->onodes) {
+        {
+          std::lock_guard<std::mutex> l(o->flush_lock);
+          o->flush_txns.insert(txc);
+        }
+
         for (auto& p : o->blob_map.blob_map) {
            p.bc.finish_write(txc->seq);
        }
@@ -4733,8 +4738,8 @@  void BlueStore::_txc_write_nodes(TransContext *txc, KeyValueDB::Transaction t)
     dout(20) << "  onode " << (*p)->oid << " is " << bl.length() << dendl;
     t->set(PREFIX_OBJ, (*p)->key, bl);

-    std::lock_guard<std::mutex> l((*p)->flush_lock);
-    (*p)->flush_txns.insert(txc);
+    /*std::lock_guard<std::mutex> l((*p)->flush_lock);
+    (*p)->flush_txns.insert(txc);*/
   }