Re: [PATCH 1/4] arm64: alternative: wait for other CPUs before patching

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Dec 10, 2021 at 02:49:59PM +0000, Catalin Marinas wrote:
> On Fri, Dec 03, 2021 at 10:47:20AM +0000, Mark Rutland wrote:
> > In __apply_alternatives_multi_stop() we have a "really simple polling
> > protocol" to avoid patching code that is concurrently executed on other
> > CPUs. Secondary CPUs wait for the boot CPU to signal that patching is
> > complete, but the boot CPU doesn't wait for secondaries to enter the
> > polling loop, and it's possible that patching starts while secondaries
> > are still within the stop_machine logic.
> > 
> > Let's fix this by adding a vaguely simple polling protocol where the
> > boot CPU waits for secondaries to signal that they have entered the
> > unpatchable stop function. We can use the arch_atomic_*() functions for
> > this, as they are not patched with alternatives.
> > 
> > At the same time, let's make `all_alternatives_applied` local to
> > __apply_alternatives_multi_stop(), since it is only used there, and this
> > makes the code a little clearer.
> 
> Doesn't the stop_machine() mechanism wait for the CPUs to get in the
> same state before calling our function or we need another stop at a
> lower level in the arch code?

The stop_machine() logic doesn't wait on the way in; it queues some work on
each CPU sequentially to *enter* the stop function (in this case
__apply_alternatives_multi_stop()), but there's no existing logic to ensrue
that all CPUs have entered by some point. On the way out, stop_machine() waits
for all CPUs to *exit* the stop function before returning.

We need to synchronize on the way into __apply_alternatives_multi_stop() to be
sure that no CPUs are executing bits which might be patched -- portions of
stop_machine() itself, anything necessary to dequeue the work, or anything that
might be running before the CPU spots the queued work.

Thanks,
Mark.

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