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Date:   Fri, 6 Apr 2018 18:22:11 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Yury Norov <ynorov@caviumnetworks.com>
Cc:     James Morse <james.morse@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Will Deacon <will.deacon@arm.com>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Christopher Lameter <cl@linux.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Alexey Klimov <klimov.linux@gmail.com>,
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Subject: Re: [PATCH 1/5] arm64: entry: isb in el1_irq
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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.