rxrpc rcu bug

The rxrpc rcu bug tag covers a Linux kernel vulnerability (CVE-2026-31642) in the RxRPC networking subsystem. The issue is a concurrency flaw where an RxRPC call was removed from the global rxnet->calls list using list_del_init() instead of an RCU-safe deletion primitive. Under concurrent access, particularly when /proc/net/rxrpc/calls is being read, this can corrupt the reader's list traversal, potentially causing hangs or other instability. This is not a remote-code-execution bug but a kernel list-handling problem affecting systems using the RxRPC protocol. Discussions focus on the technical details of the RCU bug, the incorrect list primitive, and the conditions that trigger the issue.