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Date:   Tue, 10 May 2022 18:12:35 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Rik van Riel <riel@fb.com>
Cc:     "song@kernel.org" <song@kernel.org>,
        "joe.lawrence@redhat.com" <joe.lawrence@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
        "live-patching@vger.kernel.org" <live-patching@vger.kernel.org>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        Kernel Team <Kernel-team@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "pmladek@suse.com" <pmladek@suse.com>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched
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On Wed, May 11, 2022 at 12:46:32AM +0000, Rik van Riel wrote:
> On Tue, 2022-05-10 at 17:37 -0700, Josh Poimboeuf wrote:
> > On Wed, May 11, 2022 at 12:35:11AM +0000, Rik van Riel wrote:
> > > On Tue, 2022-05-10 at 23:57 +0000, Song Liu wrote:
> > > > 
> > > > So, if we come back to the same question: is this a bug (or a
> > > > suboptimal
> > > > behavior that worth fixing)? If so, we are open to any solution
> > > > that 
> > > > would also help PREEMPT and/or non-x86 arches. 
> > > > 
> > > Using the preempt notifiers during KLP transition should
> > > work equally well for PREEMPT and !PREEMPT. It also does
> > > not insert any additional code into the scheduler while
> > > there is no KLP transition going on.
> > 
> > As I've been saying, this is not going to work for PREEMPT because,
> > without ORC, we can't reliably unwind from an IRQ handler, so the
> > kthread won't get patched.
> > 
> Isn't the sched_out preempt notifier always run in
> process context?
> 
> What am I missing?

Maybe it's technically process context at that point.  But the important
point is that the call to the scheduler via preempt_schedule_irq()
originates from the "return from interrupt" path.

So the state of the interrupted task's stack is unknown.  For example it
could have been interrupted before the frame pointer prologue.  Or in a
leaf function which doesn't use frame pointers.

-- 
Josh