Andy Lutomirski <l...@kernel.org> writes:
>> + * Invoked by all exception/interrupt IDTENTRY handlers which are not
>> + * returning through the paranoid exit path (all except NMI, #DF and the IST
>> + * variants of #MC and #DB).
>
> The paranoid-exit bit is not really relevant. The important part is
> which stack we're on. See below.
Right. I amended the comment to that effect.
>> + */
>> +void noinstr idtentry_exit(struct pt_regs *regs)
>> +{
>> + lockdep_assert_irqs_disabled();
>
> How about:
>
> #ifdef CONFIG_DEBUG_ENTRY
> WARN_ON_ONCE(!on_thread_stack());
> #endif
Made this if (IS_ENABLED()) ..
>> + instr_begin();
>> + /* Tell the tracer that IRET will enable interrupts */
>> + trace_hardirqs_on_prepare();
>
> Why is trace_hardirqs_on() okay above but not here? Is it that we
> know we weren't RCU-quiescent if we had preemption and IF on? But
> even this code path came from an IF-on context. I'm confused. Maybe
> some comments as to why this case seems to be ordered so differently
> from the !preempt_count() case would be helpful.
Added commentry. Updated patch below.
Thanks,
tglx
8<-----------------
Subject: x86/entry/common: Provide idtentry_enter/exit()
From: Thomas Gleixner <t...@linutronix.de>
Date: Thu, 26 Mar 2020 16:28:52 +0100
Provide functions which handle the low level entry and exit similar to
enter/exit from user mode.
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
arch/x86/entry/common.c | 100 ++++++++++++++++++++++++++++++++++++++++
arch/x86/include/asm/idtentry.h | 3 +
2 files changed, 103 insertions(+)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -510,3 +510,103 @@ SYSCALL_DEFINE0(ni_syscall)
{
return -ENOSYS;
}
+
+/**
+ * idtentry_enter - Handle state tracking on idtentry
+ * @regs: Pointer to pt_regs of interrupted context
+ *
+ * Invokes:
+ * - lockdep irqflag state tracking as low level ASM entry disabled
+ * interrupts.
+ *
+ * - Context tracking if the exception hit user mode.
+ *
+ * - RCU notification if the exception hit kernel mode
+ *
+ * - The hardirq tracer to keep the state consistent as low level ASM
+ * entry disabled interrupts.
+ */
+void noinstr idtentry_enter(struct pt_regs *regs)
+{
+ if (user_mode(regs)) {
+ enter_from_user_mode();
+ } else {
+ lockdep_hardirqs_off(CALLER_ADDR0);
+ rcu_irq_enter();
+ instrumentation_begin();
+ trace_hardirqs_off_prepare();
+ instrumentation_end();
+ }
+}
+
+/**
+ * idtentry_exit - Common code to handle return from exceptions
+ * @regs: Pointer to pt_regs (exception entry regs)
+ *
+ * Depending on the return target (kernel/user) this runs the necessary
+ * preemption and work checks if possible and required and returns to
+ * the caller with interrupts disabled and no further work pending.
+ *
+ * This is the last action before returning to the low level ASM code which
+ * just needs to return to the appropriate context.
+ *
+ * Invoked by all exception/interrupt IDTENTRY handlers which are not
+ * returning through the paranoid exit path (all except NMI, #DF and the IST
+ * variants of #MC and #DB) and are therefore on the thread stack.
+ */
+void noinstr idtentry_exit(struct pt_regs *regs)
+{
+ lockdep_assert_irqs_disabled();
+
+ if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
+ WARN_ON_ONCE(!on_thread_stack());
+
+ /* Check whether this returns to user mode */
+ if (user_mode(regs)) {
+ prepare_exit_to_usermode(regs);
+ } else if (regs->flags & X86_EFLAGS_IF) {
+ /* Check kernel preemption, if enabled */
+ if (IS_ENABLED(CONFIG_PREEMPTION)) {
+ /*
+ * This needs to be done very carefully.
+ * idtentry_enter() invoked rcu_irq_enter(). This
+ * needs to undone before scheduling.
+ *
+ * Preemption is disabled inside of RCU idle
+ * sections. When the task returns from
+ * preempt_schedule_irq(), RCU is still watching.
+ *
+ * rcu_irq_exit_preempt() has additional state
+ * checking if CONFIG_PROVE_RCU=y
+ */
+ if (!preempt_count()) {
+ instrumentation_begin();
+ rcu_irq_exit_preempt();
+ if (need_resched())
+ preempt_schedule_irq();
+ /* Covers both tracing and lockdep */
+ trace_hardirqs_on();
+ instrumentation_end();
+ return;
+ }
+ }
+ /*
+ * If preemption is disabled then this needs to be done
+ * carefully with respect to RCU. The exception might come
+ * from a RCU idle section in the idle task due to the fact
+ * that safe_halt() enables interrupts. So this needs the
+ * same ordering of lockdep/tracing and RCU as the return
+ * to user mode path.
+ */
+ instrumentation_begin();
+ /* Tell the tracer that IRET will enable interrupts */
+ trace_hardirqs_on_prepare();
+ lockdep_hardirqs_on_prepare(CALLER_ADDR0);
+ instrumentation_end();
+ rcu_irq_exit();
+ lockdep_hardirqs_on(CALLER_ADDR0);
+ } else {
+ /* IRQ flags state is correct already. Just tell RCU */
+ rcu_irq_exit();
+ }
+}
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -7,6 +7,9 @@
#ifndef __ASSEMBLY__
+void idtentry_enter(struct pt_regs *regs);
+void idtentry_exit(struct pt_regs *regs);
+
/**
* DECLARE_IDTENTRY - Declare functions for simple IDT entry points
* No error code pushed by hardware
