lkml.org 
[lkml]   [2018]   [Apr]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/5] arm64: entry: isb in el1_irq
On Fri, Apr 06, 2018 at 07:54:02PM +0300, Yury Norov wrote:
> In general, kick_all_cpus_sync() is needed to switch contexts. But exit from
> userspace is anyway the switch of context. And while in userspace, we cannot
> do something wrong on kernel side. For me it means that we can safely drop
> IPI for all userspace modes - both normal and nohz_full.

This *may* be true, but only if we never have to patch text in the
windows:

* between exception entry and eqs_exit()

* between eqs_enter() and exception return

* between eqs_enter() and eqs_exit() in the idle loop.

If it's possible that we need to execute patched text in any of those
paths, we must IPI all CPUs in order to correctly serialize things.

Digging a bit, I also thing that our ct_user_exit and ct_user_enter
usage is on dodgy ground today.

For example, in el0_dbg we call do_debug_exception() *before* calling
ct_user_exit. Which I believe means we'd use RCU while supposedly in an
extended quiescent period, which would be bad.

In other paths, we unmask all DAIF bits before calling ct_user_exit, so
we could similarly take an EL1 debug exception without having exited the
extended quiescent period.

I think similar applies to SDEI; we don't negotiate with RCU prior to
invoking handlers, which might need RCU.

> If it's correct, for v3 I would suggest:
> - in kick_all_cpus_sync() mask all is_idle_task() and user_mode() CPUs;
> - add isb() for arm64 in do_idle() path only - this path doesn't imply
> context switch.

As mentioned in my other reply, I don't think the ISB in do_idle()
makes sense, unless that occurs *after* we exit the extended quiescent
state.

Thanks,
Mark.

\
 
 \ /
  Last update: 2018-04-06 19:22    [W:0.208 / U:0.796 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site