os/bluestore: fix race between SharedBlobSet::lookup and SharedBlob::put

A                             B
SharedBlobSet::lookup()
  takes lock
  nref is not 0
                              SharedBlob::put()
                                --nref
returns SharedBlobRef,
  ++nref
                                takes cache lock
                                SharedBlobSet::remove
                                  takes lock
                                  removes
                                deletes SharedBlob

-> A ends up with a ref to deleted SharedBlob

Fix by verifying that nref is still zero in SharedBlobSet::remove(),
while we are holding the SharedBlobSet::lock.  The lock ensures that we
have increased the ref for the lookup before entering remove, so we can
verify that nref is still zero before removing it.  If not, we have
raced, and put() bails out and does nothing.

Fixes: http://tracker.ceph.com/issues/36526
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 020bd7b5f38a82d9eef5e25e6f4a4dd12b066915)

Conflicts:
	src/os/bluestore/BlueStore.h

diff --git a/src/os/bluestore/BlueStore.cc b/src/os/bluestore/BlueStore.cc
index 07dbe7cae33d2a75f55cd6d8b053946a0966c797..9ea6bc7ae644d7316a40966de35aa5d5024e1570 100644 (file)
--- a/src/os/bluestore/BlueStore.cc
+++ b/src/os/bluestore/BlueStore.cc
@@ -1612,8 +1612,10 @@ void BlueStore::SharedBlob::put()
       if (coll_snap != coll) {
        goto again;
       }
-      coll_snap->shared_blob_set.remove(this);
-
+      if (!coll_snap->shared_blob_set.remove(this, true)) {
+       // race with lookup
+       return;
+      }
       bc._clear(coll_snap->cache);
       coll_snap->cache->rm_blob();
     }

diff --git a/src/os/bluestore/BlueStore.h b/src/os/bluestore/BlueStore.h
index a3878779d6181caa61b97f593293c575f258a671..7715ada7e55ce45ca2af4590cb2a9c8becdc71b6 100644 (file)
--- a/src/os/bluestore/BlueStore.h
+++ b/src/os/bluestore/BlueStore.h
@@ -445,15 +445,19 @@ public:
       sb->coll = coll;
     }
 
-    void remove(SharedBlob *sb) {
+    bool remove(SharedBlob *sb, bool verify_nref_is_zero=false) {
       std::lock_guard<std::mutex> l(lock);
       assert(sb->get_parent() == this);
+      if (verify_nref_is_zero && sb->nref != 0) {
+       return false;
+      }
       // only remove if it still points to us
       auto p = sb_map.find(sb->get_sbid());
       if (p != sb_map.end() &&
          p->second == sb) {
        sb_map.erase(p);
       }
+      return true;
     }
 
     bool empty() {