[PATCH v4 3/4] arm64: alternatives: Remove READ_ONCE() usage during patch operation

Will Deacon
2020-11-03T12:18:37Z

In preparation for patching the internals of READ_ONCE() itself, replace
its usage on the alternatives patching patch with a volatile variable
instead.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/alternative.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 73039949b5ce..a57cffb752e8 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -21,7 +21,8 @@
#define ALT_ORIG_PTR(a) __ALT_PTR(a, orig_offset)
#define ALT_REPL_PTR(a) __ALT_PTR(a, alt_offset)

-static int all_alternatives_applied;
+/* Volatile, as we may be patching the guts of READ_ONCE() */
+static volatile int all_alternatives_applied;

static DECLARE_BITMAP(applied_alternatives, ARM64_NCAPS);

@@ -205,7 +206,7 @@ static int __apply_alternatives_multi_stop(void *unused)

/* We always have a CPU 0 at this point (__init) */
if (smp_processor_id()) {
- while (!READ_ONCE(all_alternatives_applied))
+ while (!all_alternatives_applied)
cpu_relax();
isb();
} else {
@@ -217,7 +218,7 @@ static int __apply_alternatives_multi_stop(void *unused)
BUG_ON(all_alternatives_applied);
__apply_alternatives(&region, false, remaining_capabilities);
/* Barriers provided by the cache flushing */
- WRITE_ONCE(all_alternatives_applied, 1);
+ all_alternatives_applied = 1;
}

return 0;
--
2.29.1.341.ge80a0c044ae-goog