[RFC PATCH v2 1/4] arm64/entry: Make debug_exception_enter/exit() noinstr

From: Hongyan Xia <[email protected]>

Commit 879a6754d3d11e30af24b7dc486f561510d62641 ran into a crash because
debug_exception_enter/exit() triggered page faults caused by perf dwarf
call graph tracing. That patch was a band-aid on top.

Instead of trying to band-aid all possible paths that can happen during
instrumentation or perf tracing, simply force-inline these functions and
use the notrace variant of preempt handling to avoid all the complexity.

Drop the RCU_LOCKDEP_WARN(): arm64_enter_el1_dbg() runs first and
enters NMI context via ct_nmi_enter(), so RCU is always watching by the
time debug_exception_enter() runs.

Also mark __preempt_count_add/sub() __always_inline so the preempt
count updates fold into the noinstr callers instead of becoming
outlined calls to instrumentable text.

Signed-off-by: Hongyan Xia <[email protected]>
---
 arch/arm64/include/asm/preempt.h |  4 ++--
 arch/arm64/kernel/entry-common.c | 13 ++++---------
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index 932ea4b62042..326f221c3f56 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -41,14 +41,14 @@ static inline bool test_preempt_need_resched(void)
 	return !current_thread_info()->preempt.need_resched;
 }
 
-static inline void __preempt_count_add(int val)
+static __always_inline void __preempt_count_add(int val)
 {
 	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
 	pc += val;
 	WRITE_ONCE(current_thread_info()->preempt.count, pc);
 }
 
-static inline void __preempt_count_sub(int val)
+static __always_inline void __preempt_count_sub(int val)
 {
 	u32 pc = READ_ONCE(current_thread_info()->preempt.count);
 	pc -= val;
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index ceb4eb11232a..d5c1d6defe29 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -292,20 +292,15 @@ static __always_inline void fpsimd_syscall_exit(void)
  * accidentally schedule in exception context and it will force a warning
  * if we somehow manage to schedule by accident.
  */
-static void debug_exception_enter(struct pt_regs *regs)
+static __always_inline void debug_exception_enter(struct pt_regs *regs)
 {
-	preempt_disable();
-
-	/* This code is a bit fragile.  Test it. */
-	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
+	preempt_disable_notrace();
 }
-NOKPROBE_SYMBOL(debug_exception_enter);
 
-static void debug_exception_exit(struct pt_regs *regs)
+static __always_inline void debug_exception_exit(struct pt_regs *regs)
 {
-	preempt_enable_no_resched();
+	preempt_enable_no_resched_notrace();
 }
-NOKPROBE_SYMBOL(debug_exception_exit);
 
 UNHANDLED(el1t, 64, sync)
 UNHANDLED(el1t, 64, irq)
-- 
2.47.3