Andy Lutomirski <l...@kernel.org> writes:
> On Tue, May 19, 2020 at 11:58 AM Thomas Gleixner <t...@linutronix.de> wrote:
>> Which brings you into the situation that you call schedule() from the
>> point where we just moved it out. If we would go there we'd need to
>> ensure that RCU is watching as well. idtentry_exit() might have it
>> turned off ....
>
> I don't think this is possible. Once you untangle all the wrappers,
> the call sites are effectively:
>
> __this_cpu_write(xen_in_preemptible_hcall, true);
> CALL_NOSPEC to the hypercall page
> __this_cpu_write(xen_in_preemptible_hcall, false);
>
> I think IF=1 when this happens, but I won't swear to it. RCU had
> better be watching.
>
> As I understand it, the one and only situation Xen wants to handle is
> that an interrupt gets delivered during the hypercall. The hypervisor
> is too clever for its own good and deals with this by rewinding RIP to
> the beginning of whatever instruction did the hypercall and delivers
> the interrupt, and we end up in this handler. So, if this happens,
> the idea is to not only handle the interrupt but to schedule if
> scheduling would be useful.
>
> So I don't think we need all this RCU magic. This really ought to be
> able to be simplified to:
>
> idtentry_exit();
>
> if (appropriate condition)
> schedule();
This is exactly the kind of tinkering which causes all kinds of trouble.
idtentry_exit()
if (user_mode(regs)) {
prepare_exit_to_usermode(regs);
} else if (regs->flags & X86_EFLAGS_IF) {
/* Check kernel preemption, if enabled */
if (IS_ENABLED(CONFIG_PREEMPTION)) {
....
}
instrumentation_begin();
/* Tell the tracer that IRET will enable interrupts */
trace_hardirqs_on_prepare();
lockdep_hardirqs_on_prepare(CALLER_ADDR0);
instrumentation_end();
rcu_irq_exit();
lockdep_hardirqs_on(CALLER_ADDR0);
} else {
/* IRQ flags state is correct already. Just tell RCU */
rcu_irq_exit();
}
So in case IF is set then this already told the tracer and lockdep that
interrupts are enabled. And contrary to the ugly version this exit path
does not use rcu_irq_exit_preempt() which is there to warn about crappy
RCU state when trying to schedule.
So we went great length to sanitize _all_ of this and make it consistent
just to say: screw it for that xen thingy.
The extra checks and extra warnings for scheduling come with the
guarantee to bitrot when idtentry_exit() or any logic invoked from there
is changed. It's going to look like this:
/*
* If the below causes problems due to inconsistent state
* or out of sync sanity checks, please complain to
* l...@kernel.org directly.
*/
idtentry_exit();
if (user_mode(regs) || !(regs->flags & X86_FlAGS_IF))
return;
if (!__this_cpu_read(xen_in_preemptible_hcall))
return;
rcu_sanity_check_for_preemption();
if (need_resched()) {
instrumentation_begin();
xen_maybe_preempt_hcall();
trace_hardirqs_on();
instrumentation_end();
}
Of course you need the extra rcu_sanity_check_for_preemption() function
just for this muck.
That's a true win on all ends? I don't think so.
Thanks,
tglx
