x86/traps: Stop using ist_enter/exit() in do_int3()

#BP is not longer using IST and using ist_enter() and ist_exit() makes it
harder to change ist_enter() and ist_exit()'s behavior.  Instead open-code
the very small amount of required logic.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200225220217.150607679@linutronix.de

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7ffb6f4..c0bc9df 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -572,14 +572,20 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
                return;
 
        /*
-        * Use ist_enter despite the fact that we don't use an IST stack.
-        * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
-        * mode or even during context tracking state changes.
+        * Unlike any other non-IST entry, we can be called from a kprobe in
+        * non-CONTEXT_KERNEL kernel mode or even during context tracking
+        * state changes.  Make sure that we wake up RCU even if we're coming
+        * from kernel code.
         *
-        * This means that we can't schedule.  That's okay.
+        * This means that we can't schedule even if we came from a
+        * preemptible kernel context.  That's okay.
         */
-       ist_enter(regs);
+       if (!user_mode(regs)) {
+               rcu_nmi_enter();
+               preempt_disable();
+       }
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
        if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
                                SIGTRAP) == NOTIFY_STOP)
@@ -600,7 +606,10 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
        cond_local_irq_disable(regs);
 
 exit:
-       ist_exit(regs);
+       if (!user_mode(regs)) {
+               preempt_enable_no_resched();
+               rcu_nmi_exit();
+       }
 }
 NOKPROBE_SYMBOL(do_int3);