[lkml]   [2019]   [Jan]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH v3 0/6] Static calls
> On Jan 11, 2019, at 1:22 PM, Josh Poimboeuf <> 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 <> 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.

 \ /
  Last update: 2019-01-11 22:37    [W:0.143 / U:3.940 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site