On Fri, Jan 11, 2019 at 09:36:59PM +0000, Nadav Amit wrote: > > On Jan 11, 2019, at 1:22 PM, Josh Poimboeuf <jpoim...@redhat.com> wrote: > > > > On Fri, Jan 11, 2019 at 12:46:39PM -0800, Linus Torvalds wrote: > >> On Fri, Jan 11, 2019 at 12:31 PM Josh Poimboeuf <jpoim...@redhat.com> > >> wrote: > >>> I was referring to the fact that a single static call key update will > >>> usually result in patching multiple call sites. But you're right, it's > >>> only 1-2 trampolines per text_poke_bp() invocation. Though eventually > >>> we may want to batch all the writes like what Daniel has proposed for > >>> jump labels, to reduce IPIs. > >> > >> Yeah, my suggestion doesn't allow for batching, since it would > >> basically generate one trampoline for every rewritten instruction. > > > > As Andy said, I think batching would still be possible, it's just that > > we'd have to create multiple trampolines at a time. > > > > Or... we could do a hybrid approach: create a single custom trampoline > > which has the call destination patched in, but put the return address in > > %rax -- which is always clobbered, even for callee-saved PV ops. Like: > > > > trampoline: > > push %rax > > call patched-dest > > > > That way the batching could be done with a single trampoline > > (particularly if using rcu-sched to avoid the sti hack). > > I don’t see RCU-sched solves the problem if you don’t disable preemption. On > a fully preemptable kernel, you can get preempted between the push and the > call (jmp) or before the push. RCU-sched can then finish, and the preempted > task may later jump to a wrong patched-dest.
Argh, I misspoke about RCU-sched. Words are hard. I meant synchronize_rcu_tasks(), which is a completely different animal. My understanding is that it waits until all runnable tasks (including preempted tasks) have gotten a chance to run. -- Josh
