MongoS blocks all requests to sharded collection

Three times on the last 48 hours, all our mongoS were deadlocked on serving requests to sharded collection.
Using out-of-prod mongoS to a sharded collection was blocked to.

During the lock, it was possible to : use db.currentOp() and show collection on the sharded database.

Restarting all mongoS was the only way to get out of this.

In attachment, a cleaned log of one mongoS during the last failure.