Merge PR #24701 into master

* refs/pull/24701/head:
	os/bluestore: fix race between SharedBlobSet::lookup and SharedBlob::put

Reviewed-by: Igor Fedotov <ifedotov@suse.com>

src/os/bluestore/BlueStore.cc

@@ -1622,8 +1622,10 @@ 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();
     }

src/os/bluestore/BlueStore.h

@@ -461,15 +461,19 @@ public:
       sb->coll = coll;
     }
 
-    void remove(SharedBlob *sb) {
+    bool remove(SharedBlob *sb, bool verify_nref_is_zero=false) {
       std::lock_guard l(lock);
       ceph_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() {