[$] Faster RCUs and lockless memory allocation

Puranjay Mohan shared some of thework he's been doing recently on improving theperformance of read-copy-update (RCU) at the 2026LinuxStorage, Filesystem, Memory-Management, and BPF Summit; his talk would havebeen nice context to have earlier in the day when Harry Yoo and AlexeiStarovoitov led a session about thenew kmalloc_nolock() function thatallows for lockless allocation from any kernel context, and which interacts withthe RCU subsystem to allow that. This article therefore covers the two sessionstogether and in the reverse order, to provide that missing context.