[PATCH] arm64: spectre: Avoid locking on v4 mitigation enable path

[Sami Tolvanen]

Hi Will,

On Wed, Feb 17, 2021 at 6:26 AM Will Deacon <will at kernel.org> wrote:
>
> The Spectre-v4 workaround is re-configured when resuming from suspend,
> as the firmware may have re-enabled the mitigation despite the user
> previously asking for it to be disabled.
>
> Enabling or disabling the workaround can result in an undefined
> instruction exception on CPUs which implement PSTATE.SSBS but only allow
> it to be configured by adjusting the SPSR on exception return. We handle
> this by installing an 'undef hook' which effectively emulates the access.
>
> Installing this hook requires us to take a couple of spinlocks both to
> avoid corrupting the internal list of hooks but also to ensure that we
> don't run into an unhandled exception. Unfortunately, when resuming from
> suspend, we haven't yet called rcu_idle_exit() and so lockdep gets angry
> about "suspicious RCU usage". In doing so, it tries to print a warning,
> which leads it to get even more suspicious, this time about itself:
>
>  |  rcu_scheduler_active = 2, debug_locks = 1
>  |  RCU used illegally from extended quiescent state!
>  |  1 lock held by swapper/0:
>  |   #0: (logbuf_lock){-.-.}-{2:2}, at: vprintk_emit+0x88/0x198
>  |
>  |  Call trace:
>  |   dump_backtrace+0x0/0x1d8
>  |   show_stack+0x18/0x24
>  |   dump_stack+0xe0/0x17c
>  |   lockdep_rcu_suspicious+0x11c/0x134
>  |   trace_lock_release+0xa0/0x160
>  |   lock_release+0x3c/0x290
>  |   _raw_spin_unlock+0x44/0x80
>  |   vprintk_emit+0xbc/0x198
>  |   vprintk_default+0x44/0x6c
>  |   vprintk_func+0x1f4/0x1fc
>  |   printk+0x54/0x7c
>  |   lockdep_rcu_suspicious+0x30/0x134
>  |   trace_lock_acquire+0xa0/0x188
>  |   lock_acquire+0x50/0x2fc
>  |   _raw_spin_lock+0x68/0x80
>  |   spectre_v4_enable_mitigation+0xa8/0x30c
>  |   __cpu_suspend_exit+0xd4/0x1a8
>  |   cpu_suspend+0xa0/0x104
>  |   psci_cpu_suspend_enter+0x3c/0x5c
>  |   psci_enter_idle_state+0x44/0x74
>  |   cpuidle_enter_state+0x148/0x2f8
>  |   cpuidle_enter+0x38/0x50
>  |   do_idle+0x1f0/0x2b4
>
> Prevent these splats by avoiding locking entirely if we know that a hook
> has already been registered for the mitigation.
>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Boqun Feng <boqun.feng at gmail.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Saravana Kannan <saravanak at google.com>
> Reported-by: Sami Tolvanen <samitolvanen at google.com>
> Fixes: c28762070ca6 ("arm64: Rewrite Spectre-v4 mitigation code")
> Signed-off-by: Will Deacon <will at kernel.org>

Thank you for the patch. I can confirm this fixes the warning for me.

Tested-by: Sami Tolvanen <samitolvanen at google.com>

Sami