[PATCH v2] arm64: spectre: Prevent lockdep splat on v4 mitigation enable path

[Will Deacon]

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 running __cpu_suspend_exit() with RCU watching.

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>
Suggested-by: "Paul E . McKenney" <paulmck at kernel.org>
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>
---

v2: Use RCU_NONIDLE() instead of eliding the spinlock

 arch/arm64/kernel/suspend.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index a67b37a7a47e..d7564891ffe1 100644
--- a/arch/arm64/kernel/suspend.c
+++ b/arch/arm64/kernel/suspend.c
@@ -119,7 +119,7 @@ int cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
 		if (!ret)
 			ret = -EOPNOTSUPP;
 	} else {
-		__cpu_suspend_exit();
+		RCU_NONIDLE(__cpu_suspend_exit());
 	}
 
 	unpause_graph_tracing();
-- 
2.30.0.478.g8a0d178c01-goog