[PATCH] arm64/entry: Don't disable preemption in debug_exception_enter() with RT kernel

[Waiman Long <longman@redhat.com>]

Commit d8bb6718c4db ("arm64: Make debug exception handlers visible from
RCU") introduces debug_exception_enter() and debug_exception_exit()
where preemption is explicitly disabled. With a PREEMPT_RT debug kernel,
the following bug report can happen.

  BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 15255, name: gdb_app
  preempt_count: 1, expected: 0
  RCU nest depth: 0, expected: 0
  1 lock held by gdb_app/15255:
  #0: ffff10007f41b7d8 (&sighand->siglock){..}-{3:3}, at: force_sig_info_to_task+0x34/0x130
  Preemption disabled at:
  [<ffff800080081ea8>] debug_exception_enter+0x18/0x70
    :
  Call trace:
  dump_backtrace+0xac/0x130
  show_stack+0x1c/0x24
  dump_stack_lvl+0xa0/0xe0
  dump_stack+0x14/0x2c
  __might_resched+0x178/0x230
  rt_spin_lock+0x58/0x120
  force_sig_info_to_task+0x34/0x130
  force_sig_fault+0x58/0x80
  arm64_force_sig_fault+0x44/0x70
  send_user_sigtrap+0x5c/0xa0
  brk_handler+0x38/0x5c
  do_debug_exception+0x78/0x110
  el0_dbg+0x50/0x1e0
  el0t_64_sync_handler+0x114/0x150
  el0t_64_sync+0x17c/0x180

Fix that by blocking the preempt_disable()/preempt_enable_no_resched()
calls when CONFIG_PREEMPT_RT is enabled.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/arm64/kernel/entry-common.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index c7a23f7c2212..191441b22b7c 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -290,15 +290,17 @@ static __always_inline void fpsimd_syscall_exit(void)
 }
 
 /*
- * In debug exception context, we explicitly disable preemption despite
- * having interrupts disabled.
+ * In debug exception context, we explicitly disable preemption except for
+ * PREEMPT_RT kernel as rt_spin_lock() can be called.
+ *
  * This serves two purposes: it makes it much less likely that we would
  * 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)
 {
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 
 	/* This code is a bit fragile.  Test it. */
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
@@ -307,7 +309,8 @@ NOKPROBE_SYMBOL(debug_exception_enter);
 
 static void debug_exception_exit(struct pt_regs *regs)
 {
-	preempt_enable_no_resched();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable_no_resched();
 }
 NOKPROBE_SYMBOL(debug_exception_exit);
 
-- 
2.54.0