Upsert is not atomic when inserting

Create a file named upsert.js with the following content: upsert.js var i; for (i=0; i<10; ++i) { db.upsert.update({a:1}, {$inc:{b:1}}, true ); } On a Mongo 3.0 WiredTiger setup, run these commands in bash: mongo --eval "db.upsert.drop()"; for((i=0;i<100;i++)); do mongo upsert.js & done; mongo --eval "db.upsert.find().forEach(function(d){printjson(d);})"; This produces something like this: { "_id" : ObjectId("55f3359949f88a6cd8a11d7a"), "a" : 1, "b" : 997 } { "_id" : ObjectId("55f3359949f88a6cd8a11d7b"), "a" : 1, "b" : 1 } { "_id" : ObjectId("55f3359949f88a6cd8a11d7c"), "a" : 1, "b" : 1 } { "_id" : ObjectId("55f3359949f88a6cd8a11d7d"), "a" : 1, "b" : 1 } It should produce something like this: { "_id" : ObjectId("55f3359949f88a6cd8a11d7a"), "a" : 1, "b" : 1000 }