On Fri, 4 Jan 2019 17:50:18 +0000 Mark Rutland <mark.rutland@xxxxxxx> wrote: > At Linux Plumbers, I had a conversation with Steve Rostedt, and we came > to the conclusion that (withut heavyweight synchronization) patching two > NOPs at runtime isn't safe, since a CPU might have executed the first > NOP as a NOP before another CPU patches both instructions. So a CPU > might execute: > > NOP > BL ftrace_regs_caller > > ... rather than the expected: > > MOV X9, X30 > BL ftrace_regs_caller > > ... and therefore X9 contains some UNKNOWN value, rather than the > original LR value. > > I wonder if we could solve that by patching the kernel at build-time, to > add the MOV X9, X30 in place of the first NOP. If we were to do that, we > could also update the addresses to pooint at the second NOP, simplifying > the changes to the runtime code. You can also patch it at boot up when there's only one CPU running, and interrupts are disabled. -- Steve
