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    SubjectRe: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier
    On Thu, Nov 05, 2020 at 06:02:49PM -0500, Qian Cai wrote:
    > On Thu, 2020-11-05 at 22:22 +0000, Will Deacon wrote:
    > > On Fri, Oct 30, 2020 at 04:33:25PM +0000, Will Deacon wrote:
    > > > On Wed, 28 Oct 2020 14:26:14 -0400, Qian Cai wrote:
    > > > > The call to rcu_cpu_starting() in secondary_start_kernel() is not early
    > > > > enough in the CPU-hotplug onlining process, which results in lockdep
    > > > > splats as follows:
    > > > >
    > > > > WARNING: suspicious RCU usage
    > > > > -----------------------------
    > > > > kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
    > > > >
    > > > > [...]
    > > >
    > > > Applied to arm64 (for-next/fixes), thanks!
    > > >
    > > > [1/1] arm64/smp: Move rcu_cpu_starting() earlier
    > > > https://git.kernel.org/arm64/c/ce3d31ad3cac
    > >
    > > Hmm, this patch has caused a regression in the case that we fail to
    > > online a CPU because it has incompatible CPU features and so we park it
    > > in cpu_die_early(). We now get an endless spew of RCU stalls because the
    > > core will never come online, but is being tracked by RCU. So I'm tempted
    > > to revert this and live with the lockdep warning while we figure out a
    > > proper fix.
    > >
    > > What's the correct say to undo rcu_cpu_starting(), given that we cannot
    > > invoke the full hotplug machinery here? Is it correct to call
    > > rcutree_dying_cpu() on the bad CPU and then rcutree_dead_cpu() from the
    > > CPU doing cpu_up(), or should we do something else?
    > It looks to me that rcu_report_dead() does the opposite of rcu_cpu_starting(),
    > so lift rcu_report_dead() out of CONFIG_HOTPLUG_CPU and use it there to rewind,
    > Paul?

    Yes, rcu_report_dead() should do the trick. Presumably the earlier
    online-time CPU-hotplug notifiers are also unwound?

    Thanx, Paul

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