Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu()

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> On Tue, May 19, 2020 at 2:20 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> Unless I've missed something, the effect here is that #PF hitting in
> an RCU-watching context will skip rcu_irq_enter(), whereas all IRQs
> (because you converted them) as well as other faults and traps will
> call rcu_irq_enter().

The only reason why this is needed for #PF is that a kernel mode #PF may
sleep. And of course you cannot sleep after calling rcu_irq_enter().

All other interrupts/traps/system vectors cannot sleep ever. So it's a
straight forward enter/exit.

> Once upon a time, we did this horrible thing where, on entry from user
> mode, we would turn on interrupts while still in CONTEXT_USER, which
> means we could get an IRQ in an extended quiescent state.  This means
> that the IRQ code had to end the EQS so that IRQ handlers could use
> RCU.  But I killed this a few years ago -- x86 Linux now has a rule
> that, if IF=1, we are *not* in an EQS with the sole exception of the
> idle code.
>
> In my dream world, we would never ever get IRQs while in an EQS -- we
> would do MWAIT with IF=0 and we would exit the EQS before taking the
> interrupt.  But I guess we still need to support HLT, which means we
> have this mess.

You always can dream, but dont complain about the nightmares :)

Thanks,

        tglx