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

[Mark Rutland]

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.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Andre Przywara <andre.przywara at arm.com>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: James Morse <james.morse at arm.com>
Cc: Joey Gouly <joey.gouly at arm.com>
Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/alternative.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 3fb79b76e9d9..4f32d4425aac 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -21,9 +21,6 @@
 #define ALT_ORIG_PTR(a)		__ALT_PTR(a, orig_offset)
 #define ALT_REPL_PTR(a)		__ALT_PTR(a, alt_offset)
 
-/* Volatile, as we may be patching the guts of READ_ONCE() */
-static volatile int all_alternatives_applied;
-
 static DECLARE_BITMAP(applied_alternatives, ARM64_NCAPS);
 
 struct alt_region {
@@ -193,11 +190,17 @@ static void __nocfi __apply_alternatives(struct alt_region *region, bool is_modu
 }
 
 /*
- * We might be patching the stop_machine state machine, so implement a
- * really simple polling protocol here.
+ * Apply alternatives, ensuring that no CPUs are concurrently executing code
+ * being patched.
+ *
+ * We might be patching the stop_machine state machine or READ_ONCE(), so
+ * we implement a simple polling protocol.
  */
 static int __apply_alternatives_multi_stop(void *unused)
 {
+	/* Volatile, as we may be patching the guts of READ_ONCE() */
+	static volatile int all_alternatives_applied;
+	static atomic_t stopped_cpus = ATOMIC_INIT(0);
 	struct alt_region region = {
 		.begin	= (struct alt_instr *)__alt_instructions,
 		.end	= (struct alt_instr *)__alt_instructions_end,
@@ -205,12 +208,16 @@ static int __apply_alternatives_multi_stop(void *unused)
 
 	/* We always have a CPU 0 at this point (__init) */
 	if (smp_processor_id()) {
+		arch_atomic_inc(&stopped_cpus);
 		while (!all_alternatives_applied)
 			cpu_relax();
 		isb();
 	} else {
 		DECLARE_BITMAP(remaining_capabilities, ARM64_NPATCHABLE);
 
+		while (arch_atomic_read(&stopped_cpus) != num_online_cpus() - 1)
+			cpu_relax();
+
 		bitmap_complement(remaining_capabilities, boot_capabilities,
 				  ARM64_NPATCHABLE);
 
-- 
2.30.2